Well, he may be the worst QB who has actually played. There are a lot of QB's on and off rosters, on practice squads, etc who have never played a down of football in the NFL and probably never will. They still make a living, and Peterman is probably better than a chunk of those guys.
Peterman is a good teammate and probably does a good job in the meeting room and helping to prepare the starter, so coaches and starting QB's like having him around. That's all. Nobody actually wants him to play, just like nobody ever wants their 3rd string / practice squad QB to play.