In multimedia patents, it's common for non-pool members to sue people. But not pool participants. E.g. There are now several cases against H.264 users, and there has been litigation against MP3 users in the past, but I'm not aware of any cases where the plaintiff was a pool member.
Recently in MPEG-LA v. Alcatel (a case where a pool member merged with a non-pool member and spun off the non-pool patents to a subsidiary to avoid the pool obligations) the courts even denied a company the ability to shield themselves from the pool obligations using incorporation stunts.