I'm pretty conservative with my use of the ignore button. I tend to ignore only trolls. If someone repeats themselves endlessly without saying anything new and just ignores comments made as rebuttals for instance. I might ignore someone if they are a brand new user and immediatley post something extremely controversial and anti-libertarian. They're not really interested in talking. They just show up to drop a bomb and waste people's time.
I think I've ignored like 2 people and I think I even un-ignored someone after a short while.