With a sloping top tube, the seat stays and seat tube are a bit shorter, so the frame weighs a tiny bit less than a flat top tube. Maybe the trend started so that manufacturers could claim they trimmed 50 grams off their frame. However, you'll need a longer, beefier seatpost, so it probably cancels out.
I like the classic flat top tube. Someone said Cannondale now has a sloping top tube bike !?!? If so, WRITE & COMPLAIN !!
+1 on the comment about leaning on your top tube before & after races !