Not sure I understand the question. Or maybe it's that I don't have the answer (no idea if people, in general or specifically, are using them). I would say it would be hard to TOTALLY ignore someone because posters often quote other posters.
I use them. there are a (very, very, very) small number of posters on this board that I simply don't like to read or associate with. I have found they never had any thing important to say in any conversation and just generally are there to disrupt the thread and even deliberately try to get it closed.
That being said I don't think its a big deal if I happen to see an 'ignored poster's' post quoted in another reply.
I just don't want to bother reading someone that I know is going to be inflammatory or annoying (I have a very low threshold for both).
And in reality (and to avoid anyone getting all up in arms over this ) the actual number of ignored users I have is two.
Of course after posting this maybe I'll get on a few people's lists