Two item overlooked (I think) so far in this thread.

1. For the sport rider, a healthy portion of the performance/effeciency gains of the clipless pedal system is in the shoe. Biking shoes have especially stiff sole/arch areas. This provides a gain in effeciency due to lower power losses as the foot flexes and eventually fewer problems with the physiological structure of the foot as it trys to distribute power from the ankle area to to ball of the foot. (I think this is sore planters tendons)

2. Even if you pedal normally (push only) most of the time, there will be instances where a little burst of power will get you up the hill or over the top of the hill more easily. This may not be an effeciency improvement but you will feel better for it.

It possible for sport riders to use the shoe without cleats or clipless pedals. Easy walking touring shoes or mountain shoes without mud grabbing soles work very well on platform pedals. Look up the Shimano M020 or M032 style of shoe to see what I mean. This style of shoe allows you to walk normally (feels just slightly strange because the sole does not flex much) but is still stiff enough to be more effecient and to reduce foot pain.