One thing I forgot to add:
Flats = fun
Clips = power
Of course your results may vary but thats my experience when switching back and forth.
One point made that I'm not sure I agree with is the Sam Hill argument. We don't know he (and riders like him) would have done if he chose to be clipped in for most of his career. I would guess he would only get faster. I mean, you are not taking away advantages by riding clipped in, you are mostly always gaining (except really muddy or steep courses). You can pedal really efficiently on flats no doubt, but going to clips doesn't take that away, ya know?
Who knows? I know its a pointless argument to make since it hasn't happened.
One thing I have always stated is that if you take one rider who is proficient at both, the clipped in rider will almost always be faster. You can ride clipped in the exact same way as being on flats. The only difference is you have extra gains in the pedal stroke, plus an easier ability to keep your feet on in the rough.
:)