Can't have players you regularly only utilise 50% of the time, as would put to much pressure on other midfielders with rotations.
Better off playing Thommo mainly in th SANFL where he can do his job of working with young midfielders & give him the odd game in the AFL, where he will be fresh to give it his all.
If Thommo starts the season and plays extremly well, do you keep playing him until he falls apart or holds togetther, or drop a player who is performing ?