They don't really need to employ lawyers at all. It's just an insurance club. They need an actuary, an accountant and a clerk. They can pay law-firms to do the lawyering, and contract researchers to dig up prior art. That way, the main costs arise on a per-case basis - only the office staff are fixed costs.

That's seem to make a startup LESS attractive companies, no? One bad case and the whole thing runs out of money.

This is the kind of service you'd want provided by somebody "too big to fail".

True, but the goal is to fight patent trolls or non practicing entities. Not take on legitimate cases of patent infringement against Oracle or Intel or Apple

Winning an straightforward case in the American justice system can be quite expensive, especially when the plantiff goes judge shopping (which they all do, it's part of what makes a patent troll a patent troll).