H. G. Muller wrote on Sun, Mar 24, 2013 09:08 AM UTC:
Allowing promotion to powerful pieces like Rook completely changes the character of the game. It really doesn't have a Shatranj flavor anymore. The strategic objective becomes promoting Pawns, like in FIDE Chess. While in Shatranj promoting usually gets you nowhere.
Upgrading the Queen from Ferz to Commoner might already have this effect, if the Pawns now promote to this type of Queen.