From what I've seen, they seem to be popular with the Triathalon crowd. Unless you were planning on using them to carry an additional two bottles (as opposed to the one or two you could normally fit in the frame) I don't see the advantage. The Triathaletes I've seen with them used them in lieu of the frame cages. Perhaps there's some logic to this I'm not aware of. More aero?

another possible use would be to have the ability to easily carry two bottles when your frame size/geometry doesn't really permit you to easily use two cages. An example would be the RS2 frame in the 49c size. I haven't built mine up yet, but others have stated that cages on the ST interfere with the FD. My first solution to this will be to try to use a side loading cage, and if that doesn't work I'm look then to the seat post mount soultion.