All the RBs you listed also get 25+ touches per game. The Saints just aren't build like that. The reason the Saint offense works so well is because the ball is spread, and that's part of Bush's role, is to open everybody else up. I'm not going to look it up, but I'd be willing to bet that when Bush is on the field, Drew's comp% is way up there. If you look at Bush's YPC (3.98) over his career, it isn't elite, but it's not quite the bottom of the barrel either. I'd also be willing to argue that Bush's pass protection is better than CJ or AD.
Bottom line, you may not get more production wise from Bush than a CJ or AD, but you get a lot more versatility. If he can only stay healthy...