The main advantage of clipless pedals is not the ability to add force on the upstroke. It's the fact that I don't need to expend effort keeping my feet on the pedals. The higher the cadence you like to ride, the more you feel the benefit. My current commuter has flat pedals, and there are times (especially during shifts) where one of my feet will slide out of position on the pedal. That just isn't an issue with clipless pedals.