Ignoring adverse selection, having insurance means "There is someone with a shitton of money and lawyers ready to go toe to toe". This means there are better targets. Much like a home security system: easy enough to circumvent, but easier to just move to the next house.

Of course, if all of those who sign up are willfully infringing legitimate(ish) patents, the calculus changes a bit. Perhaps the insurance company would insure against particular patents?