I am under the impression that it is an easier defense (at least "we patented this because that's what we're doing, so either your patent which postdates ours is the same, so it's invalid, or not, so we're not infringing it"), even if you don't get to countersue a non-troll competitor. That is, while you still have to go to court against a troll, having your own duplicate patent means you're less likely to lose, which is even more onerous than just paying for lawyer time.

(I don't base this on any particular courtroom experience with this, though, so maybe I'm totally wrong.)