For me and clipless - it isn't the upstroke where I gain power, it is at the bottom of the stroke where I can pull my foot back. This has been referred to the "scraping mud off of your shoe" simulation. You just cannot pull your foot back as efficiently and as directly without being clipped to you pedal. You can to an extent with a tightly fit pair of toe clips and straps, but with regular platforms it is difficult. The only time I gained benefit from upstroke is when I am climbing standing, and even then very little. Keeping this into consideration, I still like using platforms for what they are - the ease to just get and go is nice.