The injunction against feeding trolls is one part logical and one part rhetorical. 

The logical part consists in the implication that feeding the troll misrepresents the troll's contributions.  In addressing a troll's view one implies that it strongly represents the dialetical situation, when, in fact, it doesn't (largely because the troll doesn't himself believe his on view)–Iron manning, in other words (making the troll appear stronger than he is). This is a variation on the injunction against weak manning: picking on trolls is nut picking,

Rhetorically, addressing trollish criticism puts one on the defensive.  One isn't making one's argument so much as defending oneself against criticism.  The public mind can only listen for so long, so chances are your responding to trolls diminishes your ability to make your own arguments.

Advantage trolls.  The advantage is especially acute nowadays, because the intellectual side of one of the two parties in our lovely two-party system consists almost entirely in trolls.  Someone ought to explain this to this guy:

Of course, not all right-wing pundits spew hate. But the ones who do are the ones we liberals dependably aggrandize. Consider the recent debate over whether employers must cover contraception in their health plans. The underlying question — should American women receive help in protecting themselves from unwanted pregnancies? — is part of a serious and necessary national conversation.

Any hope of that conversation happening was dashed the moment Rush Limbaugh began his attacks on Sandra Fluke, the young contraceptive advocate. The left took enormous pleasure in seeing Limbaugh pilloried. To what end, though? Industry experts noted that his ratings actually went up during the flap. In effect, the firestorm helped Limbaugh do his job, at least in the short term.

But the real problem isn’t Limbaugh. He’s just a businessman who is paid to reduce complex cultural issues to ad hominem assaults. The real problem is that liberals, both on an institutional and a personal level, have chosen to treat for-profit propaganda as news. In so doing, we have helped redefine liberalism as an essentially reactionary movement. Rather than initiating discussion, or advocating for more humane policy, we react to the most vile and nihilistic voices on the right.

He's right on the rhetorical points, but on the logical point, Limbaugh and his ilk represent current Republican thinking in both style and substance.  Being high-minded about them, I think, just leaves their arguments unanswered.  Answering their arguments cedes rhetorical ground. 

It's a trap.  Anyone know a way out?