None of you circus-monkees mentioned the one aspect of trolling that has any virtue to it (although "The Question" came close)"
DISCUSSION TROLL
Said troll is deeply interested in a particular topic and desires to move (shit I can't type... 2nd glass of wine,...) the discussion beyond the confines of "groupthink" towards discussion of all options, not just the politically correct ones.
Often, the discussion troll may take the adversarial (devil's advocate) side to argue with someone he agrees with for the purposes of strengthening his target's viewpoints.
This goes back farther than internet trolling to various discussion media where consensus is formed. I would imagine this type is older than Adam.
Soemone using a topic I was dear to: muslims beheading 4 year olds because they were a "threat" caused me to join TW.
Shit... my spelling's getting worse. better end it... Nothing better for true public concensuys than a discussion troll.