I like the concept. I just wish they'd throw those of us with zero musical talent a bone.
Archeage did this by using MML, so even us musical illiterate could compose songs. :\
I don't mind having the manual way, but I'd like to also have an automated way. SE could even chip in and release MML versions of all their songs.
BTW: MML is a standard, so I don't believe there's any licenses for the language to worry about.