The ability to play custom music will be, at least for me, a HUGE bonus. While I absolutely adore Borislav Slavov and his works (not just in this game but, for example, Crysis 2), he obviously didn't create a million different tracks for the game.
So, custom music would open up a plethora of abilities for the GM to set the atmosphere of a map. I think this feature is a must-have and I really hope Larian will implement it.
Incidentally, while the latest patch did thankfully squash the music bug (it behaved strangely before and sometimes refused to play the selected track) and they introduced many scores that were previously not available for selection, they also decided to rename the majority of the tracks... This meant my large campaign which I'm designing at the moment, required a complete overhaul of music for every location, as the previous track names simply did not exist any more.
Larian, if you guys are going to rename tracks again (and hopefully add some more, like that pirate-esque battle one!), please don't rename them any more. :)