BikeJames.com - Top 3 Clipless Pedal Myths
[...] most of the advantages of clipless pedals are lost when you stand and so riders that use them tend to sit and spin a lot. Standing up to pedal is only hard if you lack the core strength and hip drive to stabilize and power from the standing position. Most riders come into mountain biking with weak cores, weak hips and serious dysfunctions from sitting all the time.
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In the hands of someone whose reality isn’t shaped by the same dysfunctions flat pedals allow you to ride every bit as hard and far; you just have to use a different technique that isn’t possible without addressing the core and hip weakness that are really at the root of the issue.

Rivbike.com - The Shoes Ruse
When elite pedalers and lousy rookie pedalers have been hooked up to machines that measure muscle activity during pedaling, the machines tell us this:
during normal pedaling at normal cadences, nobody pulls UP on the backstroke
the elite/efficient pedalers push down less on the upward moving pedal than the rookies do.
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There are some benefits to being firmly attached. Whether they make sense for you and your riding, only you can answer. Here they are:
1. in slippery conditions and vicious sprints, and when hopping the bike over a dead raccoon or up onto a curb, a connection to the pedal is a benefit.
2. When you climb a super steep short hill, you actually can pull up on the upward-moving pedal for a few strokes, and doing so helps you turn over the other pedal (get it past 12:00 and into the power part of the stroke).
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Your foot doesn't bend when you pedal a bike. It tenses and pretty much stays straight, just as it does when you walk up stairs.