So one of the problems you have to solve if you create a NATO-like network of companies, is that one of your members might decide they want to go up against Apple. Now your organization has to decide what counts as a "troll" case and what counts as "you should've known". And since some industries are at a higher risk of trolling than others, those more expensive members will inevitably drive out the safer members who foot the bills but don't benefit very much.
So in the end you have problems somewhere in between an insurance company and...NATO :p
If a NATO nation gets 'attacked' by somewhat provoking a war, NATO must ultimately make some sort of decision about whether or not to jointly act (and foot the bill).
http://www.nato.int/cps/en/natohq/topics_110496.htm
_Collective defence means that an attack against one Ally is considered as an attack against all Allies._
It has been invoked once. In 2001.
what counts as a "troll" case
Most companies avoid patent litigation in all but the most severe instances, because so many people hold patents for the most obvious of things that they might well get counter-sued by the company they're suing. Mutually Assured Destruction in patent form.
NPEs don't have this concern, so they can send out all sorts of dubious claims without fear of countersuit.
An alliance against NPEs (with a rule preventing NPEs from joining) might well be worthwhile.
"We made a batch of 10 'somethings' last year, so we are not an NPE or a troll!"
If you have deep pockets and a grudge, you would pay your legal team to try and have every single one of their patents/revenue streams invalidated.