That's a good thread that obloquy links to, worth a read.
It's got me thinking a little more about this too. It's always possible that Saruman, who was obviously a tinkerer, simply made a ring
because he could. He could have been like one of those people who loves to pull things apart just to see how they work. I know I have this urge to pull things apart and investigate them myself, which is possibly part of why I find Saruman so interesting.
But I think he did have darker purposes. I still think that one of the major qualities if not
the purpose of the Rings of Power was linked to osanwe. The Silmarillion tells how Sauron sought to control the minds and thoughts of others, particularly through giving them these Rings. We have also seen how Galadriel has considerable powers of osanwe. And we know Saruman is strong in this respect, with his voice being particularly dangerous.
Saruman, to my mind, was trying to create his own Ring to enhance his powers, but he failed to be entirely effective in his endeavour. When Sauron made the One, it is clear he had to pour an immense amount of his own power into its crafting; that the destruction of the One caused the destruction of Sauron and his realm, demonstrates just how much of his power was put into its forging. It's obvious that Saruman did not put
quite so much of his own power into the crafting of his own ring, which may be a good reason to explain why it did not work effectively.
But this ring may have had some effects and there is evidence in LotR. Saruman controlled a large army, and not only an army, but he also had gained control of the Dunlendings, and had allies in Rohan and the Shire. Who knows what he could have acheived with more time? He was not in Middle earth for anywhere near the time that Sauron had, and he did not have the remnants which Morgoth left behind for Sauron to pick up and use; he was working from scratch to build his power. He was also easily able to deceive such perceptive minds as those of Gandalf, Galadriel and Elrond, all true Ringbearers. Again this may suggest a purpose to his Ring, and that may have been to cloak or cloud the minds of the bearers of the Three - something which he may have sought to do as he wanted to find the One and did not want the Council to perceive his aims.
There is another reason why Saruman may have failed to make his Ring more effective. Maybe a Ring cannot stand 'alone'. Just as much of the potential of a Palantir is lost without another Palantir to communicate with, perhaps a ring of power needs some 'kin' which are in affinity with it in order for it to have genuinely wide ranging effects? And finally, it could be that the One Ring was so powerful because of
where it was forged. When it was cast back, it caused the destruction of its maker and his realm, suggesting that this was a cataclysmic kind of 'force' - to return to source an object
created from that source. It has got me thinking about something davem said some time ago, that Mount Doom may be a 'tap' to some kind of power within Middle earth, maybe to the Light itself? If Saruman (and for that matter, Celebrimbor) lacked this power during his forging, then the ring/s produced could
never have the same terrifying potential of the One.