Here's an interesting read, a report from the Cycling Science Symposium & Expo. Several things in it, including a session on "private numbness."

At this symposium, an expert bike fitter did say that clipless pedals helped his business because it locked in cyclists to riding for extended periods in a fixed, repetitive position. This increased injuries unless one had a very good fit. They didn't address the pedaling efficiency issue.

http://www.velonews.com/tech/report/...s/11555.0.html