It's usually not the patent trolls. It's companies like Apple that they need to protect against. I guess patent trolls don't want you to go out of business as long as they get the "protection money". Besides you have a situation were a country as a whole declaring war on a patent troll. That could create good PR and a favorable judgement.
How does a patent bank protect against trolls who cannot be countersued as they are not infringing?
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>