That is a big point. Trolls. Some people do it just as a game. They think it is funny to wind people up. Get some perverted pleasure from it.
Knowing when to walk away is great. Knowing when to step back and NOT have the last word is great. I learnt that a few years back. Knowing that I didn't have to always show that "I was right" let me enjoy forums so much more.
It also lets the idiots show themselves up for the idiots they really are. After a while everyone else has left and they are standing in that room all on their own.
XKCD has it right as always...
http://xkcd.com/386/