The troll may in particular choose to praise one of the leader of a community, thus casting doubt on his ability to recognise a troll for what it is. This may threaten the stability of the community, while people wonder over the lack of clarity of their leader. Publicly voicing the doubts will further comfort the troll in the efficiency of his flattery. Flattering a rather minor participant is less likely to be effective.
As the troll generates bad will in the community, SelectiveFlattery may have a different effect. When the flatterer is disreputable, the flattery is undesirable. Positive or even neutral comments made by the praised person are likely to be perceived as active support to the troll from the rest of the community. The targets of the praise may try to disassociate themselves from it, and even viciously attack the flatterer. This keeps the conflict going, and may serve to confuse people who stumble into it at this point; why attack someone who is saying nice things about you?
So the best strategy may be to ignore flatteries.