The case for/against clipless, or even straps, is sort of summed up in this piecethis piece from the Rivendell Bicycles website. They mention studies, albeit without citing the exact source, that actually pulling up on the pedal is extremely unlikely, except maybe on short uphill or sprint bursts, and so being attached to the pedal is far from being a must. And they suggest, without supporting evidence, that it may actually make you a better cyclist, because without attachment your legs have to learn to go in circles, and not simply be taken along for the ride.
There is also the issue with positioning your foot on the pedal: all that careful messing around with the cleats so that the ball of the foot is exactly over the pedal axle. But then there is that other blog from Joe Friel, who is an evidence nut, suggesting that there is no proper support for that being best, and that it may be more efficient to put your cleat under the arch.
Joe's blog, together with the (again, inadequately referenced) story of the Japanese competitor in Ironman New Zealand that forgot to put his shoes in the transition bag, and went to a bike personal best riding barefoot on clipless pedals, have really changed my outlook on the need for attachment to the pedals. I ride with clips on my fixie, but am more and more considering changing to plain pedals and see where that takes me.
