
I am not for or against this yet, because its new, and it hasn't been done. But first off, I do not pay for my own normal-music, so im not sure I would ever want to pay for "game" music. (I still use radio, internet radio, satellite radio, etc)
There are a few ups and downs to this, some of the downsides worry me.
Upside: Obviously, more music. Some mmorpgs are very lacking in the music/sound department. This may give them incentive to do something more and go the extra mile.
Downside: Games will do the same thing they do with, "Patches". Release a game not so complete and just add music when they feel, and charge us for it. Also there was mention of a "special edition" of the music x-pac with 4 different songs, perhaps us who purchased the "collectors edition" of the game should receive this free of charge?
To wrap it up, I think its a very new idea and i'm willing to see where it goes. Personally I think after the first time listening to a track of music, we really just block it out from there on and concentrate on the game. Of course, we all have the choice not to purchase music. So at least it is not forced.

I am very neutral on it still, looking forward to hearing what others think.