From what I see, the coin system seem to be really iffy. People either have way too many coins, or they keep running out. I, being in the former, usually go ahead and finish the song anyways, even if I know I won't beat the score I'm after. This (along with the occasional DLC pack purchase) kept my coin stash really high up there with nothing to do with it. It feels like another "Career Score" counter, but we already have Blitz Cred.
But for the people who are struggling with coins, it sucks the fun out of the game since they have to constantly grind short songs (If any statistics show, Charlene is probably the most-played Blitz track ever solely for this reason). I agree with taylorlee's idea to incorporate a bunch of cosmetic items for people to go after. Maybe even release new ones weekly to keep people coming back.
Come on HMX, we got an idea here!