It won't make "music mods" more popular.
If you want to find out just how much trouble the music system can be, go check a thread called "New music coming your way soon".
It's not just slapping a music file into a folder, and telling the game to use it... the table entries for music system seem to be rather sophisticated, because the engine doesn't just play file after file, it fades them into each other according to the situation, and the fading is timed to happen at appropriate times...
Besides, because of the fact that anyone who wants to listen their own music can already do it, I seriously doubt that anyone withing the SCP coders would want to spend their time on this kind of thing.
Everything doesn't need to be so complex and integrated. Plugging an mp3-player to Line-in
Works. So does using a low-profile mp3-player, although an external player is easier to manage during missions, as you don't need to alt-tab to the player window to change files etc.
If it works, don't try to fix it.