Although I recently switched to clipless, I've never at any time bought into the idea of "power on the upstroke". The idea is patently ridiculous; as when one pedal is in the upstroke, the other pedal is on the downstroke. As long as you are always applying force to the downstroke pedal, the upstroke will take care of itself/doesn't matter.

Some kind of foot retention is good, though- and while I like clipless well enough, I can say that it really doesn't matter to me whether I use clipless or loosely fastened straps and cages. 'Long as it keeps one's canal boats from slipping off the pedals, it's good.

I've noticed not so much as an iota of performance gain from switching to clipless. (Not that I was expecting any)