I mean really are they?
On one hand they are a pain as they “appear” to be plain argumentative. But lets look into what they really are doing.
As a rule they never share their identity and based on that they have no real importance and credibility.
Their agenda is to cause trouble and argue. By doing this they are paying with their attention, if you argue back you are paying with your attention.
Trolls will never cause you any damage or discredit you (providing you have done nothing wrong) because a. they have no identity (as above) and b. they get bored easily and will go away soon enough (most trolls are just angry, and anger gets boring after a while) – moreover, they are discrediting themselves by their own actions.
Trolls are good for you as every now and again they can challenge your thinking even if it’s not in the direction they wanted to take you in the first place.
Trolls are like a creche, you can learn by them so when entering a “real” debate you replies will be so much the better for the training trolls have given you.
Trolls keep threads alive, even old ones.
So on the whole Trolls are quite good really – other than that you have to admit some of them are really quite funny!