I see it two ways:

Pro: If you have a troll following you around trying to trash on threads (as I have had), being able to use ignore to keep them off of threads you start is a nice tool to have to keep thread content healthy.

Con: If, on the other hand, the person you are ignoring is someone who bothers you but still posts good, knowledgable content from time to time (and we have plenty of those types of members), then their being blocked from your thread can have a downfall...even if they're jerks, they still might have something useful to add.

So I think that's what we're looking at. Does the pro outweigh the con, or vice versa? Or is there a way to have our cake and eat it too?