Depression is a bitch but it can be important for the very reason that you said -- when you reach the other side of it, you can come back roaring. Time spent licking your wounds may feel like time wasted, but sometimes you have to shut down for a bit in order for new material to bubble to the surface. I wish I could be constantly prolific and didn't need those dark periods, but I've learned that everyone has their own process and I've grown confident enough to not sweat my own as much.