First, you won't be able to get insurance for this case, because it predates your policy. A bit like getting fire insurance when your house is already on fire.
Second, an insurance company won't just write you a blank cheque for legal fees, they will take over the case and look for the cheapest way out, which will probably be settlement.
Hence, specific insurance against patent trolls could work by effectively taking away the easy targets for patent trolls. It's a bit like home-insurance companies that offer discounts on good locks to keep out thieves. Everyone except for the criminals are better off afterwards.
I don't know if the second point is a big deal. Even if they choose to settle, you've still mitigated your risk without going to court. And I suspect a patent litigation insurance agency is going to be motivated to negotiate very small settlements and/or actually fight it out in court, lest they gain a reputation as the company that hands out license fees.