No I dont use it.
Why not? Because I think people are a lot like me -- and I dont like to, and, in fact, wont register, just to post a passing comment. But then, I am generally blog-unfriendly and have very sites that I actually read with any routine.
What happens if you disable it?
The planets collide.
Seriously, what do you think happens? People can post comments without registering.
If you ask me, the purpose of that option, in the beginning was to thwart anonymous commenter spam. That was, of course, before Askimet, and spam karma, and the, literally, 100000's of other ways one can manage spam.
Since spam is manageable now (notice I didnt say stoppable) it's really not necessary to force people to register _just_ to comment.