My $.02. I've ridden platforms w/out retention of any type, platforms w/ clips, straps & flat shoes , the same with cleated bike shoes & clipless. I have no scientific proof, but I have to believe retention is more efficient than no retention. I also think clipless is more efficient than clips & cleated shoes, but the main advantage is comfort. Pulling the straps tight enough was just uncomfortable especially on long rides. Now, my livelihood does not depend on whatever efficiency gains (if any!) I get from using clipless over no retention, so it really comes down to what's more fun. I don't commute and rarely stop when I ride, so for me clipless is the way to go. I feel a much better connection to the bike than w/out retention. If I was commuting or riding to Star$s I might have a different answer.

Dan