You know, as much as I agree that the option should be there, I don't think I would use it at all. Call me a traditionalist, but I think you need to experience the game with its original soundtrack. So much work goes into creating the OST for a game, and it is tailored to match certain highs and lows within the game. Substituting other music would take away from that experience, personally.
it would be awsome to have the option to listen to the music they select BUT. Listening to ur own music would be even better. I would like to listen to music while kicking some heros ass or some villains ass in DC universeor while playing Disgea 3' or the new UMVC3. listening to there music all the time is wack.
I was thinking , it would be an incentive to have music unlimited available on home or in game. Everyone could choose their own music , unless you were in someone's personal space( apartment) ,Or everyone in a game for instance COD , could choos a music style by team . It would be a win, win, win, situation , for playstation,music unlimited, and for gamers .