I really like the Minoura Double Cage Saddle Mount:
http://www.trisports.com/midocasamo.html
Leaves a nice big gap that accomodates a saddle pouch nicely. It also only costs $15. I have it on 3 bikes and it's been great. You will have to supply your own bottle cages (which are cheap).
I don't have them for aerodynamics but simply to augment my fluid carrying capacity. Also, one of my bikes only has a single seat tube bottle cage mount that's taken by my pump.