I'm disinclined to like an "ignore feature" for a couple of reasons. As stated above, it lets users stop listening to anyone they disagree with, thereby stopping debate cold, and as this site is all about debate, it seems counterintuitive.
On the otherside, if someone registers and starts prosyletizing, I may be very inclined to ignore them, but if they're truly able to understand their mistake and begin evolving into a better debater, I've cut myself off from their better arguments. Based on what others are saying, I may be able to realize they've gotten better and "unignore" them, but for an ignore feature to really work, I would think it would also ignore responses to someone you've ignored...or am I wrong there?
Basically, how would I know if someone's gotten better and deserves to be listened to if I can't see what they're doing? There are a few posters here, I won't name names without a bribe, that I just skip over. I read the responses to them, however, and if I see a promising line of discussion, I'll go back and read the original post. This could be difficult to do, ignoring and unignoring the same person over and over.
Anyway, just my 2 cents.