Maybe it's just me, but I've had my best fishing on flies that are pretty ratty looking. If it isn't bent or a bare hook, I'll hang on to it.

I have one "fly" on my bench that went through a bluefish frenzy, and even though it has only a few strands of white poly left hanging on the hook, it kept on catching and catching until the school was gone. I retired that one, but I keep it on the tying bench just because.