Pretty sure that someone used one a few years ago (American, possibly Zabriskie?) I suppose the logic is that you don't have to reach for a bottle and there may also be some slight aero advantage by smoothing out the junction between helmet and back.
A quick rider in my club used one in a 100mile TT but it didn't work well for him. The hump interfered with his helmet so had to wear the pouch on his front, which caused him to overheat in the hot conditions. I guess a pro rider would have the helmet's tail adjusted to fit though.
Camelbak Racebak - useful for 50 and 100 mile TTs, but the water does tend to heat up so you use that first and then reach for the bottle in the latter stages. For some riders it is more beneficial due to their position, but as maurice said it can also be a problem against the pointy helmet.
The hydration methods seen in the pro peloton have been varied - some have used bottles behind the seat, others standard bottle on frame, and others (notably Garmin) with the Racebak. Each probably has its benefits but they'll be rider specific.