I'd still do the Tribunal if there were no IP reward. I do it because I enjoy it - I want people to be punished for trolling. I do judge each case. I'm the kind of person who's supposed to do doing the Tribunal, not the kids who just punish every case. The only problem with them auto-punishing is that if enough of them do it, they'll outweigh people who're actually trying to judge.
So what about other people who actually judge cases? Would you stop without the IP (which is dismal, anyway)?
In fact, the IP is really only worth it if you are auto-punishing. 50 IP for 10 minutes is good. But it's not great for 25 minutes. It's more fun than a quick surrender, yes, but you're also not likely to get all 50 IP.