Originally Posted by cyccommute
No because the seatpost isn't that heavy.
Neither are a few centimeters of frame tubing. Since metal frame tubing is usually butted and seatposts usually are not, a cm of extra seatpost tends to be heavier than a cm of extra frame tubing. So while you =might= save more frame tubing than you add seat post, the seat post you add is heavier. When you account for this, the net difference in weight and stiffness going from a sloping to horizontal top tube is trivial to none.
If there were a true advantage in weight and stiffness of a sloping top tube, then in a sport where these minor differences are actually relevant it would be a bit hard to explain how that sport's top event had been won seven of the last eight times without it. (It could even be eight for eight, I haven't kept up on the controversy.)