My personal rationale for not liking the "ignore" feature is that I personally like to believe that we are all mature enough to be able to ignore someone without having to actually hide their posts. And if we are not, then there are much bigger issues that need to be addressed. An ignore feature just puts a band-aid on a bigger problem. Personally, I would like to see a community of mature posters, who show respect to each other, and act like responsible adults.
I understand that this may be unrealistic, and perhaps an "ignore" feature would help some people maintain their heads at times, but I just really do not think it is asking too much for people to simply ignore people on their own. I am sure everyone does it all the time in real life without too much of an issue, so why change that on a message board.
At the same time, as moderators, we may be able to do a better job of policing the line of baiting versus "negative nellies", but I don't think that is a good reason to put in an ignore feature either.