The purpose is to limit intelligent debate.
I agree- it is used far too much. A while back I witnessed an FR brawl that I was disgusted by. Some poster had the cyber snot beat out of them as a "troll". I read the poster's point, and would not have declared him or her a troll, I would have discussed/debated his/her points. It was one of the ugliest things I had witnessed. Usually, I would have weighed in, but I lacked the time to do so, and to uphold my position against the onslaught of FReepers thoroughly verbally and emotionally abusing the poster. It was just awful.
The purpose is to limit intelligent debate.
I think this also qualifies as the result of such behaviors.