I think Tressel's approach right now is perfect. Continue to allow the freshman to grow into his role, giving him more and more of the offense every game. Holmes is still getting the ball more than Ginn, as it should be, considering their experience level.
To play the devil's advocate however, Holmes has put the ball on the ground a lot this year (whereas Ginn has not yet done so with his limited touches).
Next year, with an experienced line, a seasoned defense, and perhaps one or two top recruits to split carries with pittman our offense could be quite potent. Don't forget the 45, 52, 45, 50 points tressel put up during 4 of the 7 games in 2002. Then clarett (among others) got hurt and everything went downhill.
If we are able to return to an offensive powerhouse, it will prevent defenses from keying on Holmes like they do now. When that happens, especially with a seasoned Ginn and other receivers, Holmes could have a breakout year. So don't disregard his pro stock next year in terms of him possibly leaving early.