Interesting discussion thus far. One thing I started thinking about as I was making some adjustments to my wife's mixte is why the designers decided to run the twin diagonal tubes back to the rear dropout rather than attaching them to the seat tube or just using a single sloping downtube. According to what many are saying here, a frame should be stiffer by positioning the top tube lower on the seat tube. But in the mixte design (at least the Peugeot that I'm looking at) the twin diagonals go from the head tube to the rear drop outs and aren't connected in any way to the seat tube. So those designers apparently felt that it is not necessary to brace the seat tube at all as it's only attached at the bottom bracket and the thin seat stays. Now granted mixte frames aren't likely to be put through the same level of stress as a racing frame but still, it does make one wonder about the argument that the top tube is there to somehow reduce flexing of the seat tube. Apparently its primary function is to support the loads on the head tube.