Originally Posted by
PiLigand
Let's break this down, engineering style.
Unless you've got one of those funny (and pretty damn cool, imo) elliptical crank sets, your crank and drive train in general are radially symmetrical. So really, they don't care which 'direction' you're pulling/pushing, just that energy is going from you to it. (In reality, since it's a polar [radial] system, you are only really pushing in one direction the whole time, you're just using different muscle groups to accomplish it. So, if consider using the exact same amount of energy on platform vs clipless, you just get to spread that energy payment over different muscle groups, which helps to keep you from exhausting yourself too quickly.
Also, it should help you maintain a more consistent power output. Like I said, the crank doesn't really care which "direction" you push, but you're not using the same radial system as it. So when using platform, you can only push down, which only lets you use one quarter(ish) of the crank rotation per arm, or about half the rotation in total. Pulling/pushing in other directions uses - again, assuming the same amount of energy - the same energy spread more consistently over the crank.
As for clipping/unclipping, I still have the occasional trouble unclipping really quickly, but it's a necessary evil IMO.
You have some assumptions in there that might be unsupported. You can't assume that spreading the work among different muscles groups improves efficiency, because those muscles may differ in their mechanical and chemical efficiency . So you might end up underusing your most efficient muscles, and overusing some inefficient ones. You also can't assume that steady power output is more efficient than pulsatile output concentrated on the downstroke. In fact pro cyclists exert much more effort on the downstroke than the upstroke and it seems to work for them.