Well originally all I saw was a thread that was getting a lot of posts and when I click on it I see that it was someone on my ignore list. Yes I know that you could hover over a thread and if it was someone on ignore it wouldn't display the first few lines of a thread. However when just opening up a section of the WoW forums if I see an interesting topic I click on it, now some of the topics that might be a rare gem just because someone on my ignore list finally made a decent thread I would miss it.
Best example would be (without naming names) a person in WoW general that creates many threads, 9 out of 10 are garbage and they make so many it was best for me to ignore them but 1 out of 10 times they'd make a thread that would have good discussion in it despite the presence of the OP.