According to analysis of the most recent PlayStation 3 Software Development Kit, version 1.80 now provides developers with a method of streaming music off the user's hard drive. While this costs 12MB to implement, it's assumed that the cost will decrease in upcoming SDK releases. And while this still takes a good amount of memory, custom soundtrack support is the most wanted feature that led to the In-Game XMB Overlay request.
The SDK also drastically reduces memory usage to 48MB from 52 and video memory usage dropped from 34 to 24. The question does come up. Did Sony give developers this choice too late in the game? They are currently already working on an In-Game XMB overlay that will make use of the music function--so what's the point of this now?
Chances are these bits of information to be taken out in the next update to reduce the usage will primarily be to take this option away from developers when the time for the In-Game XMB Overlay comes.