The problem with changing this for nobles with units, is that battles only run at turn change, and that turns are really fundamental to the game.
It would also seriously unbalance the game to enable a noble to recruit, march one turn at a time to their border region, drop their men as militia, then head back to the Capital as quickly as they can recruit.
Certain class combinations are invalid to prevent exactly these sorts of abuses, including priest/infil and priest/trader, and the general prohibition against priests having any sort of unit or paraphernalia.
