[Boromir88]

Sauron could 'pour' his power into things, he did so with the One Ring. However, Sauron was not like Morgoth who just 'squandered' his power into everything, to control/corrupt everything, and in doing so weakening himself. Sauron took his far lesser power than Morgoth had and put it into a Ring; actually causing himself to be 'enhanced'. So, as obloquy said before, Sauron just giving out power gains to people doesn't really follow the way Sauron did things.

Let's put this Letter 210 into a little context. It was written to Forest J. Ackerman, and is Tolkien's commentary on film treatment of his books (specifically towards Morton Zimmerman). To say that Tolkien didn't like Zimmerman is rather understating things. Tolkien completely rips into Zimmerman's script, and despised it so badly that he refused to comment on his script from ROTK saying:

Part III .... is totally unacceptable to me, as a whole and in detail. If it is meant as notes only for a section of something like the pictorial length of I and II, then in the filling out it must be brought into relation with the book, and its gross alterations of that corrected. If it is meant to represent only a kind of short finale, then all I can say is: The Lord of the Rings cannot be garbled like that.

And as said, Tolkien 'not liking' Zimmerman is an understatement:

"I should say Zimmerman...is quite incapable of excerpting or adapting the 'spoken words' of the book. He is hasty, insensitive, and impertinent."

This was Tolkien's comments after receiving the screenplay from Zimmerman and a few months later he would write to Ackerman Letter 210.

So, what is the point? As obloquy has argued this supposed power boost only appears in one place, and in a letter that Tolkien seemed to be very nit picky, even to the point of a rant in his strong 'disapproval' of Zimmerman's script. There is already one part in that letter where Tolkien contradicts something that he wrote in LOTR:

20. The Balrog never speaks or makes any vocal sound at all. Above all he does not laugh or sneer. .... Z may think that he knows more about Balrogs than I do, but he cannot expect me to agree with him.

With a terrible cry the Balrog fell forward...~The Bridge of Khazad-dum

What Tolkien consciously wrote in his letters, after writing LOTR, is interesting insight but has to be used a little cautiously. Especially if it's a Letter to someone he probably didn't like (as Ackerman got the project of trying to make LOTR into a movie going) and ranting about someone's screenplay he definitely didn't have any respect for.

Let's also look at other parts of the Letter:

9. Leaving the inn at night and running off into the dark is an impossible solution of the difficulties of presentation here (which I can see). It is the last thing that Aragorn would have done. It is based on a misconception of the Black Riders throughout, which I beg Z to reconsider.

Than the quote continues with what's being debated...the whole 'added demonic force.' #9 is completely about the primary weapon of the Nazgul...their fear. There is no mentioning about the Witch-King's 'combat prowess,' or abilities as a sorcerer, it's Tolkien commenting that Aragorn would never have left Bree at night, because at night the Nazgul's 'fear' is greatly increased.

(Why can we use the whole part about the Nazgul's 'fear' in this Letter, while questioning the 'accuracy' of other parts of the Letter? Because, Tolkien commenting on their primary weapon - fear - does not contradict anything that he wrote in his story. We can see the Nazgul rely on fear and have a great advantage to those who 'fear' them, but to those who do not fear them, the Nazgul don't seem to be much of a threat).

Then again, we know from the Hunt for the ring, UT, that Sauron conveyed to the WK that secrecy must be abandoned - that is, before Frodo set out on his quest.~Raynor

Yet, he is still not in the role that he was at in Pelennor Fields. Sauron was hoping to keep the purpose of the Ringwraiths on the 'DL':

Thus Sauron tested the strength and preparedness of Denethor, and found them more than he had hoped. But that troubled him little, since he had used little force in the assault, and his chief purpose was that the coming forth of the Nazgul should appear only as part of his policy of war against Gondor.~Unfinished Tales; Hunt for the Ring

Because he wanted secrecy at this time...

The Lord of Morgul therefor led his companions over Anduin, unclad and unmounted, and invisible to eyes...~ibid

However, as you mention, Sauron eventually comes to want speed more than secrecy:

For Sauron had now learned of the words of prophecy heard in Gondor, and the forth going of Boromir, of Saruman's deeds, and the capture of Gandalf. From these things he concluded indeed that neither Saruman nor any of the Wise had possession yet of the Ring, but that Saruman at least knew where it might be hidden. Speed alone would now serve, and secrecy must be abandoned.~ibid

This however doesn't mean the Witch-King was willing and able to reveal his full power...this just means forget sneaking around secretly, Sauron wanted to find out where the Ring was and get it as soon as possible. As Radagast tells Gandalf about the Nine being around:

"I have an urgent errand," he said. "My news is evil." Then he looked about him, as if the hedges might have ears. "Nazgul," he whispereed. "The Nine are abroad again. They have crossed the River secretly and are moving westward. They have taken the guise of riders in black."~The Council of Elrond

Which is where the Witch-King (and his cronies ) start running all over the place to find the Ring (to Saruman in Orthanc, to Grima in Rohan, to Sarn Ford, and eventually to The Shire). In the meantime of doing this the Witch-King wasn't like how Mr. Jackson portrayed that went rushing through slicing off everyone's head...nor was he commanding an army. He was looking for information, hence why he paid a visit to Saruman and Grima...and if people stood in his way that he had the capability of defeating (as the Dunedain had attempted) well then comes the head slicing.

This is what I meant by my comment about his 'rise in status.' Not that the Witch-King grew in Sauron's heirarchy (he was already at the top), but that his role is different. At the start, him and his merry gang, are searching for the Ring where he doesn't need to use (nor show his full power), but at Pelennor he is in the position where he has to show his full power.

Why would the Witch-King conceal his full power and not go around displaying his 'grandeur?' Why does Gandalf conceal his full power when he's with his friends? In Gandalf's case, he is restricted from revealing his full power to Men and Elves. With the Witch-King he is looking for information about the Ring, he doesn't want to go around shattering house doors, or slicing off the heads of possible informants:

In that hour the Wormtongue came near to death by terror; but being inured to treachery he would have told all that he knew under less thread.~Hunt for the Ring

At Pelennor Fields, the situation has changed, he no longer is out trying to gather information about the Ring, his job is to burn Minas Tirith to the ground for his Master. So he is in a position where he has to use his full ability. Grond wasn't breaking down the gate (it had been pounding for quite a while) so the Witch-King comes by to assist Grond in breaking down the gate. Once the gate is broken everyone flees before him, except for one - Gandalf - The Witch-King may not have known Gandalf's full abilities, but this isn't the first time they've met...Gandalf proved to be difficult against the Nine before, but also the very fact that he stands alone, ready to fight while everyone else fled must be a pretty big tip off to Gandalf's power. So, the Witch-King tries to play his biggest card...fear, he laughs, mocks, lights up his sword...but after all that 'Gandalf stood unmoving' and the Witch-King leaves.

Well, if you choose to ignore the prologue, Gandalf's and Bombadil's words depicting the WK to the reader as a leader of armies .... fine.~Raynor

Those are small glimpses, little snippets, of the Witch-King's command role. He is not in that position for a majority of the story, but he was in the glimpses we get of the past:

"For even the Wise might fear to withstand the Nine, when they are gathered together under their fell chieftain. A great king and sorcerer he was of old, and now he wields a deadly fear."~The Council of Elrond

Letter 210 is the only place where if the Witch-King received a 'power uppage' than that's where it is. So, Tolkien contradicted himself, as it wasn't Sauron's nature to start boosting up the power of his servants and spreading it everywhere. Also, taking into account who the letter was written to and the context; you may argue just how accurate is this?

Or, it's as obloquy has argued...the role of the Witch-King is entirely different in the first two books than it was in the third. In the first two books the Witch-King is concealing his power because he is out looking for information, and there rarely was a need, or a purpose, to start breaking down gates and displaying his full abilities. When there was a need (for example the Nazgul crossing over the Anduin, or defeating the Rangers at Sarn Ford) than he could briefly reveal himself. However, these instances still don't require the Witch-King unleashing his full force, just enough so he could get the job done (as his task was still to look for the Ring). Where Pelennor Fields is entirely different, and we see the Witch-King (in the context of the Lord of the Rings) in a role we haven't had before. In the command of Sauron's second largest army, who's goal is to destroy Minas Tirith. Opposing him are some pesky Gondorians, but also Gandalf who has given him problems before and is the only one who stands in his way at the Gate (a gate that was dang hard to break down ). A situation which calls for the Witch-King to unleash his full bag of tricks and abilities as a sorcerer. Before Pelennor Fields, we get little comments of the Witch-King's ability as a 'great sorcerer' and his role as a leader, but now the reader actually can 'see' his power; and not just from several vague comments made by Tolkien's characters.