Not too often, riding bareback can be a strain on the horse's back. That's partly the reason why saddles were invented, to distribute the rider's weight.
Jenny
Somehow, I'm thinking like most old inventions, men invented them to cover up for poor riding. Native americans rode bareback before the explorers came over with their silly contraptions- I'm willing to bet that saddles were made to do what they still do- help strap someone up there. While western saddles can have a prupose (horns for hooking things and pulling/dragging), english saddles, to me, are just there to hold you on.
Realistically, if you are riding 'correctly' and don't have very much weight in your stirrups, a treeless saddle isn't going to distribute any more than riding bareback- no firm tree, so no pulling the whole tree down with your weight. Otherwise, I could lay a blanket over my horse and claim that it was helping to spread my weight =P
Sweat marks on saddles do show that the whole treed saddle weighs down- but take a look at them. Just how much smaller is your rear end?
Realistically, an invention as old as the saddle is not as thoughtful in origin as you're thinking. Only in the last century have we stopped being complete idiots around horses and started learning things about them- pictures of the sports they did way back when are.. cringe worthy.
and, as I said, if you know HOW to ride bareback, it can be safer for the horse than a saddle
I pride myself on being able to ride bareback well. In my mind, if a saddle has the potential to have to be strapped down from front to back (breastplates, back sinches), it's not correct. A rider alone should never have to be strapped down or held on.
It's just my way of looking at things- natural is better. But, I'm also the one who wants to ride tackless with no tricks like ropes or sticks, so I'm on the extreme end of things. I've learnt to adapt to uncomfortable withers