Plenty of people have already gotten a letter; if they all pooled their resources they'd have a decent defense fund already.
> , or maintain their membership once they're no longer in court?
This one, I agree, is harder. Best I can propose is a minimum 5 year membership once you've been sued. Or tweak it some other way so that membership is slightly cheaper, or about as much as, settling.
The ultimate protection racket! "You've got a nice product here, shame if a lawsuit happened to it..."
Wouldn't you know, it already exists: https://en.wikipedia.org/wiki/MPEG_LA
The key distinguishing factor here is the protection pool actively seeks to invalidate bad patents. It could also lobby for patent reform. By doing so, it makes patent trolling less profitable and thereby, reduces the need for itself in the future. It's like a health insurance company funding research into perfect health and immortality (but more achievable).
So why would anyone buy in before they get a letter, or maintain their membership once they're no longer in court?