Originally Posted by halfspeed
Depending upon the amount of slope and the size of the frame, what you save in frame weight and flex you may pay for in the form of a longer, heavier seat post.
Sloping top tubes are great for fitting people with short legs and long torsos, and they are great for reducing the amount of inventory bike shops need to carry because they can sell fewer sizes.
For people with longer legs and shorter torsos, a horizontal top tube means we can save the added weight and flex of a really long seat post.
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.