To respond to some earlier comments...
Cycling is a game of
efficiency and the way to do that is not through "mashing" the pedals. A smooth and efficient stroke is created by begging the down-stroke early, a strong push down, a pull back at 3 to 4 o' clock, and then unweighting the pedal in the upstroke.
Efficiency wins the race, and in the world of professional cycling everyone is at a similar level and the difference between winning and losing is often very small. Take the obvious Lance v. Jan example, both are very comparable people in terms of abilities and I believe I read that they have similar power to weight ratios (?). Point being that they are both very strong but Lance's smoother and more efficient stroke probably helped him out quite a bit.
Calves are also important in all quick bursts in cycling. In a sprint, standing climb, or jump out of a turn you should be pulling quite hard up in your stroke. If you want proof of this watch Tom Boonen in a sprint. You can see the force he puts into the upstroke on his sprint, the tail of his bike gets pulled into the air. Also watch Lance in the Alp d'Huez uphill TT from the tour. You can clearly see his calf muscles being put to work.
Also about the comment of pros having "girlie" legs. I don't know what pros you are talking about, but the ones I know have fairly large calf muscles. Look at most pros and you will see impressive and defined calves. Take Jeremy Powers, he is a Jelly Belly pro and I've heard he has to run a mtb. bottom bracket to stop his legs from hitting the top tube.
http://www.jpows.com/Images/001c9842_stdRe.jpg They look big to me, but maybe I just haven't seen a big calf before. Also many cyclists develop the V in their calves and I believe that is mostly developed by the upstroke in pedaling.
Here is an interesting article about the upstroke. I
recommend reading it.
http://www.fitness-concepts.com/CoachesCorner/May05.pdf
Lastly about the clipless pedals, their main benefit is to allow a person to apply pressure around the entire pedal stroke and we all know that. That is why we use them and a freestyle BMX'er doesn't. Both groups want a stable and non-slippery platform. They just don't need to have the ability to pull up on the upstroke like we do.