Here's what's wrong with this idea.
UserA posts some question, and after a few replies, UserB responds. The thread now takes a turn and starts discussing the points UserB has brought up. UserB has no life, so he keeps respondin to others, and pretty soon the entire thread has revolved around what UserB posted.
You happen to have UserB ignored because he's a jerk and you hate him and everything he stands for. You see this thread and think it's interesting, so you go in and start reading (And you DO read all the other posts in a thread before you respond, right?). After a while, you have absolutely no idea what's going on. People are making random replies to something you can't read, because you have UserB ignored. And you can't tell what's going on, because the forum doesn't tell you it's skipping a post, it just does and acts like UserB never posted.
It's easier to just ignore the person in your mind then in the actual software. You don't have to respond to every post that you disagree with.