i think it's much more facile/intuitive to modulate speed with light backpressure (if you're riding with someone else, for example) than to constantly pedal-brake-pedal as riding conditions change. this doesn't mean, however, that it makes sense to mash a big gear to get going, then full-out skid every time you need to slow down— if you want a big gear, a brake is nice to use; otherwise a lower gear might make more sense.