I'm suprised at this. With so many good photographers here, it strikes me as weird that so few of you use flash. With a couple of notable exceptions in this thread, you all seem to be (pretty virulently!) anti-flash. I can only assume you are thinking of on-camera automatic flash and flat, over-lit, and all around bad pictures. This is a shame. Any good photographer will tell you that flash units can and should be very useful tools. Light is the essence of what we do, and understanding and controling light, rather than merely observing it, is the key to good photography- even with rangefinder cameras. Not just for light with which to make the exposure, but for a million different creative uses, I always have at least a small flash and a longish sync cord in my bag, along with some white cards and maybe a plastic diffuser. For jobs, I'll have two or three, along with a radio tansmitter and reciever, and some reflectors, and various other light shaping tools.
I assume tht with all your available light shooting, you are aware of shadows and other areas with less light. I will also assume that at least a few of you use these areas on purpose, hopefully in creative ways. Using flash should be no less intentional or creative. I answered the poll "As needed" and while I occasionally use fill flash, I will just as often put them behind subjects to create separation, hold them off camera to get angled light, aim that at specific points for accents or to bring out features, etc. etc. You guys should get creative, and start using all the tools available to you. If photography is writing with light, you should put some ink in your pens!