I guess the thinking is that if you have enough patents, the other guy is surely infringing one (or more) of them.
That's the whole point of trolls (or Non-practicising Entities), if they don't do anything (besides sue) then they can't be countersued on the basis of patents they infringe.
<evil>The defendant could use their patent bank to sue the troll's licensees, thereby starving the troll.</evil>