I prefer a sloping TT for one very simple reason that has nothing to do with performance, ride or weight. A sloping TT allows me to clamp my bike in a workstand without moving the seatpost up and changing the saddle height.
I need two things to achieve a decent fit - a 570mm+ TT and a 150-160mmHT. Many bikes using traditional designs use 580-600mm (to top of collar) in order to make those two things work together. Assuming 190mm from the top of the saddle to the top of the clamp, and 760mm from the center of the BB to the top of the saddle, that leaves 570mm of seattube before I run into a problem. While standover in this case is not a problem, it leaves me with about 9cm exposed post. Typical Park shop clamp is 10cm. Adding a slight slope to the design renders the problem moot and I don't have to screw around with setting my saddle height every time I work on the bike.
Whenever I spec a custom, I always spec a small amount of slope. For rack bikes, I simply don't buy one if it's more than 57 CTT.
Aside from this triviality though, I don't see any advantage from the design whatsoever. And, I do think a bit of slope is pretty cool looking. Extreme compacts - no.