I wasn't pointing any fingers or saying any of you were beginners. I was simply answering the question of why it is thought that bareback can be bad for a horse.
Of course an ill-fitting saddle can do damage to a horse but I would hazard a guess that a 250 lb person that never rides can do a lot of damage bareback too. Especially if they are putting all their weight down on the horses spine.
Heck, my 1300 pound QH winces when my 170 lb boyfriend gets on him bareback. So yes I would think that a bareback rider could cause pain to a horse. If you don't think so, I suggest you take a beginner and throw them on bareback and let them run around. Look at the expression on your horse's face as the flop all over and hold on with their hands.
And again, I ride my horses bareback all the time. Just the other weekend I took Soda out with a girlfriend riding Lily and we went on an hour and a half trailride. We did plenty of flat out gallops, trotting, and cantering. Neither of the horses showed any ill effect or discomfort, but you could see when we got off that the sweat marks left by our butts were similar to what you see in a saddle with plenty of spine clearance.