Depression does fuck all either way for creativity, but what it is very good at doing is making sure you're afraid to be rid of it lest you lose that, all the while actually serving to make every creative act an uphill battle you will probably lose. Getting out from under that rock produced the finest and most productive creative phase of my life.
The myth of the tortured artist is one of the ugliest in modern culture. Go look at the lifestyle indulging that produces, and tell me that's a good route to creativity with a straight face. Because I will grant it one concession: guilt and self-recrimination can indeed be a pretty powerful quality control measure, but it is also a short jaunt from there to self-destruction and even suicide. Hemingway drank himself to death. Woolf drowned herself. I'm pretty sure you could lose count inventorying the lives of musicians who've spiraled down that hole.
As far as I'm concerned, this romanticism for depression is no less destructive and unhelpful than "pro-ana" boards; it is an enabling fiction for the mentally ill, and nothing more.