If it hadn't done this Sam would have insisted on taking him through Moria with him as he needs someone to pamper and most likely wouldn't of gone off with Frodo.
I don't think this is necessarily true...as much as Sam loved Bill, he loved Frodo more and would have chosen to go with Frodo over Sam. Also, I don't think Bill would have made it into the mines anyhow. It might have taken quite a while for the other members of the Fellowship to convince Sam to let him go, but I think it would have been accomplished in the end. The Watcher just sort of sped up the process.
As for what the Watcher was, I think it is sort of like Shelob...not particularly an agent for either side, just something malevolent lurking in an out of the way place. Sauron may have known it was there, and regarded it the same way as he did Shelob: it just made him happy to know that there was something nasty and evil out there that he didn't really have to worry about controlling. I don't think the Watcher was aware of the Ring; it just was doing what it would to any potential prey. After his battle with the Balrog, Gandalf says there are things in the deep places of the world older than Sauron. I think the Watcher is one of these things. The Watcher, if it is, as I assume, older than Sauron or at least unconnected with him, really isn't particularly good or evil...just nasty.
So if the Watcher saved the company, specifically Sam, from the insidious influence of Bill, doesnt that make it an agent for good?
Really, there's nothing in the world that's so bad that nothing it does or causes has any good consequences. If you think about it, the Ring itself is sort of an agent for good. Although it was created for evil and did cause horrible things to happen, in the end, good prevails. If the Ring hadn't been created and then destroyed, Sauron or a worse power would still be lurking somewhere.