Okay I'll give it a whirl...
1. Punt retruning is different than kick returning in a few aspects. One, it is tougher to catch the ball and make a decision on if to return it or call a fair catch or let it go if you are close to the goal line. Also the actual return is more about making people miss rather than having raw speed. Kickoff returns are based on speed but the punt returns are not. Davone Bess is better at making people miss (supposedly) and he also catches all the punts and doesn't make a mistake by letting any go (Ginn did that a few times).
2. Ginn is being used a lot at WR now so he can't do it all. We need him to be fresh and if we added punt returns to his duites, that could amount to between 5 and 10 more plays per game. If he is not conditiioned well then we may have to take away 5 to 10 plays from him on offense
\I agree with you though, I think it should be Ginn, not Bess.