I'll take a peek, though I'd assume the rationale is that your're setting everything up when you begin the song - once the song is going, it's just continuing from verse to bridge to other assorted song parts. Takes less mental focus than when you're getting the mana streams/song matrix/etc. up and running.

That said, though, I've not personally compared it to concentration drain (and exp gain) in standard spellcasting - so I'll see how it looks.

-V.

"Reject me not, sweet sounds! oh, let me live,
Till doom espy my towers and scatter them.
A city spell-bound under the aging sun,
Music my rampart, and my only one."
-Edna St. Vincent-Millay