I can't see the stiffness argument. Here's why. To simplify, we have a truss consisting of two diamond shapes sharing the seat tube as the common member and supported at back by wheel and at the front by the fork. Assume that the overall length is the same and the load is applied at the BB. Then the bike with the horizontal top tube should be stiffer since it is a deeper truss for the same length.
I can see the argument that fewer sizes are needed but at the same time most manufacturers went to thread less headsets which seems more limiting than the traditional stems. How many spacers are reasonable and still look OK?