It is especially helpful for climbing because there is more penalty at the dead spot in the pedalstroke if you don't pull up with your hamstrings to keep the momentum. Maybe the power isn't generated by the upstroke, but without the contribution it is much more difficult to generate and sustain threshold power. I'm not sure what your qualifications are here, but I've done millions of feet of climbing and have done much experimentation on the sibject.
I'm glad you've climbed x many feet, truly a feat of strength mr costanza could be proud of! I kid... Anyways, i'm glad you changed your idea about power-generation coming from the up-stroke... really though that isn't coming exclusivly from your hamstrings, since the hamstring only moves the foot backwards towards the glutes like lifting your foot off the ground when running. On a bicycle though, the hamstring motion is not part of the platform pedal's dead-zone. The dead-zone occurs simultaneously when raising your quad up and using your hip flexor and abdomen like a runner lifting their quad from perpendicular to parallel(just like you do in a roman chair) with the ground after raising their foot up...
So yes, hamstring for sweeping back but, not pulling up and clipless pedals help to keep the foot planted at low rpm as another posted says.