The League has a lot to do even in J'onn's presence. Flash can run so fast that he can travel trough time, still, he has problem facing villains who aren't know for their power levels. It all depends in the writer's creativity, it's just a matter of looking at Morrison's JLA where everyone served a purpose. And he does add to the group, he even was a great leader. And someone could correct me if I'm wrong, but he does have or have had a secret identity. And let's not forget White Martians were created from his background. He might not have a large rogues galleries, but as I already said, he works better in a group that alone, not that he couldn't work alone, as Vision or Hawkeye have shown, but his strenght will be mainly in team books