I have two bottle cages on my bike, with two 24oz bottles. On anything less than 30 miles, I usually only carry one bottle and I rarely ever finish it on the ride (in heat as high as 95deg). Anything over a planned 50 miler, I like having my 3L Camelbak. I don't have a problem with a sweaty back, I find the water stays cold longer in the pack then in the bottles, and the coolness from the water generally feels great on my back on a warm day.
I should also point out that most of my rides are in the country, where yes I go through towns with convenience stores every 10 or so miles but I would prefer not stop for water if I can help it as I don't like paying for plain water. On my typical 50+ mile route I can take a slight detour to hit up a McDonald's and refill my bottles with filtered water out of their soda fountain, but at c-stores I haven't been successful at getting my bottles refilled for free. Usually the clerk tells me they sell bottled water, even though I clearly don't need a bottle and they have a sink behind the counter. Maybe I look desperate enough to buy bottled water when I ask for water, or perhaps they think since I'm buying a snack that I'd buy a bottle of water if I had to. Having two bottles and a 3L bladder on my back just assures I won't run out of water no matter how hot it gets.