[Zigûr]

Originally Posted by jallanite

Because Sauron saw that immortal and invisible Men completely under his control would be useful servants/slaves. The Elves may not have even considered giving rings to humans. But Sauron, as it later turned out, had a different agenda than did his Elvish helpers.

Quite so, the Seven and the Nine were given out by Sauron himself after he seized them during the War of the Elves and Sauron, not by the Elves. There is that tradition in Khazad-dûm that the Ring of Durin's line was given to them directly by the Elves rather than by Sauron but it's not completely verifiable:

It was believed by the Dwarves of Durin's Folk to be the first of the Seven that was forged; and they say it was given to the King of Khazad-dûm, Durin III, by the Elven-Smiths themselves and not by Sauron, though doubtless his evil power was on it, since he aided in the forging of all the Seven. ~Durin's Folk

I wonder - did Sauron intend for the Nine to turn their bearers into Wraiths? It was certainly useful that they did, but was it part of his plan? The Seven hadn't quite worked as planned on the Dwarves (they only made them greedy and wrathful, not susceptible to external control), but maybe that was the exception due to their unusual natures, which Sauron did not fully understand. I suppose the most efficient way to use the Nine would be to have them always controlling the same nine Men who never died, and thus making them into Wraiths would seemingly be the only way to do that, and had certain perks like making them terrifying and sorcerous. I always got the impression that the Seven and the Nine were identical, which is to say sixteen Great Rings with identical powers, and that their division was based solely on race; the Seven only had strange effects because Dwarves were using them.

But Sauron gathered into his hands all the remaining Rings of Power; and he dealt them out to the other peoples of Middle-earth, hoping thus to bring under his sway all those that desired secret power beyond the measure of their kind. Seven rings he gave to the Dwarves; but to Men he gave nine, for Men proved in this matter as in others the readiest to his will. And all those rings that he governed he perverted... etc ~Of the Rings of Power and the Third Age

This would mean that Sauron had to an extent wasted seven of them on the Dwarves and might explain why he spent a certain amount of effort in the Third Age trying to recover them. Did he want to re-use them and never got around to it, or were they simply too much of a potential danger as weapons in the hands of his enemies so long as he lacked the One? It is said of the Dwarves using the Seven that:

wrath and an overmastering greed of gold were kindled in their hearts, of which evil enough after came to the profit of Sauron.

But perhaps "evil enough" was not quite the kind of top quality evil (as it were) that Sauron really wanted. It seems to me that his master plan with the Rings, to have effectively conquered all of Middle-earth in one stroke, was on the one hand a brilliant gambit which would have payed off extravagantly if properly executed and yet, evidently, contained a great deal of risk which proved to be his downfall.