I like clipless pedals for upstroke dealing. It trains other muscles, and makes me more efficient. If my technique is wrong, I feel resistance around TDC and BDC, which is fatiguing; eliminating this automatically adds twice as much power (the resistance is caused directly by my own power input, so that power both ceases to be resistance AND becomes extra input power), which is all kinds of awesome.
Of course, for that, you have to be trying. I guess this is why bicycle rollers exist; though I also do this to get up semi-challenging hills. Maybe I should get rollers for testing the shifting systems and other stuff.