Originally Posted by cyccommute
If you use a seatpost of the same diameter as can be found on most road bikes, I'd agree. I've even used those in the past on old steel mountain bikes. But my current mountain bikes all have 12 to 14" of seatpost exposed and they aren't flexy. But they are 31.0mm diameter posts, too.
So, basically, you've taken away some lightweight butted seat tube reinforced with a crossing member and replaced it with an extra heavy, straight guage piece of tubing that needs all that mass to make up for the fact that it isn't supported with a cross member and this is "better" how? For the same weight and stiffness, you can build a traditional geometry frame with stiffer tubing and a shorter seat post.
Bottom line, a sloping top tube only provides benefit in terms of fit for =some= riders and inventory reduction for the bike business. All this nonsense about "lighter and stiffer" is pretty much unwarranted marketing claims. Don't confuse the characteristics of a =frame= with those of a =bike=.