Ask HN: Would a NATO like pact against software patents work?

One of the reasons given for filing patents is that they are filed for defensive purposes. Wondering some kind of a NATO like collective defense pact has not emerged amongst startups/large software companies, where if a single company gets sued for a software patent infringement, then all other companies in the pact agree to sue the plaintiff. While this probably wouldn't work against patent trolls, wouldn't this take software patents out of play for many cases? And since I seem to hear many people/companies take the position that "software patents are evil, but we only need it for defensive purposes", would this not solve the problem cheaper? Membership fees could be in either in terms of patents or money (interchangeable). Wondering what I'm missing because at least I'm not aware something like this has emerged. (This of course assumes that software patents are detrimental to innovation in the industry)