Like someone else said, its hard to carry 70-100oz of fluid in bottles. Unless you have 7 bottle brackets. This is important when doing something like the Slick-Rock trail in Moab's heat!
- two Zéfal Magnum bottles get you to 66 oz. using the two frame cages.
- two 20 oz. Specialized BigMouth bottles, mounted on a seatpost rack (triathlon style) get you over 100.
(You could also get to 96 oz. using the same cage setup and four Polar 24 oz. bottles.)
For a long-distance, on-road ride, that'd work like a champ.
Granted, that's not great for mountain riding, though not impossible to fathom. Off-road, I'd still prefer a hydration pack, even with its ventilation issues. I, too, have tried the Camelbacks that purport to suspend the pack away from your back, but the system doesn't work particularly well in my experience.