Hmm I never really analyzed it, but now that I think about it, it's not that I lift my feet off the pedals, it's that I reposition my feet and my body for better leverage to push as hard or even harder than my weight.

I'm also not lifting upwards, so I can focus on the downstroke. With clipless pedals since I am pulling upwards, I feel like I miss pushing down hard right at the top of the stroke. With the platform pedals I'm almost pushing slightly down to keep my feet gripped on the pedals, so I am in a great position to bear down.

Anyways it seems that for me:
SPDs = good for smooth and efficient cadence (nice, even and consistent), not much force so efficient stroke important
platform = good for mashing and accelerating (anything uneven), lots of force so strong stroke important