Originally Posted by
merlinextraligh
The long stems are a function of riding smaller frame sizes. The major reason to ride a smaller frame size is that it results in a lower position, i.e. its easier to get a significant drop from the seat to the bars with a 56cm top tube, and 120mm stem, than it is with a 58 top tube and a 100mm stem.
Other reasons, stiffness and weight don't really matter for tDF riders ( you can make any frame acceptably stiff theses days, and they're all at the UCI weight limit anyway)
So the answer is aerodynamics.
As for "S turns" with pedal strokes, that happens to everyone riding a bike. Take a look at yourself with an aerial shot. You just don't notice it, without that viewpoint.
By pulling up on the handlebars at the same time you are pushing down on the pedal, you are using arm muscles to oppose the pedal stroke about the roll axis. By using the arm muscles to pull/push the bike about the roll axis-it is a way of supplementing your leg muscles with arm motion. The S-turns become necessary component of achieving that aspect when in the saddle as your body moves with the bike. If you are standing the S-turns diminish as the movement of the bike along the roll axis is fully independent of the rider.