I play blitz exclusively. I've never played in a real tournament and could probably count the number of 30-minute games I've played in my life on one hand. So obviously I can't say whether or not it hinders my normal chess skill, but I can say without a doubt that blitz has improved my tactics, openings and endgames.
It may have taken me 300 losses on a certain opening before I figured out the crucial wrong move I was making every time, but I did eventually understand it. Would I have learned it faster studying the traditional way? Probably. But I did still learn it via blitz, and blitz is more fun to me than studying.
I also believe there is much to be learned from watching stronger players in blitz matches, and perhaps even looking over their blitz matches afterward. When I watch masters play bullet or blitz, their plans unfold so fast right in front of me that I can't help but notice trends on "when to do this, when not to do that," and I can even pick up on some of the strategy/thinking that went into their moves which might not have been immediately apparent if I was just studying a master's normal game on my own.
who's that anyway?
A slap in the face to attackers everywhere.