I used to ride in jeans for my once a week lessons, but once I got a horse and was riding anywhere from 3-6 times a week, the rub marks came on full force!! Lol. Whoever said that the inner seam on jeans causes rubs was right. They do, and they hurt, Plus, with the sloppy leg, skin can get pinched between the leathers and the saddle. Ouch! Half chaps fix that problem more than breeches do, but then again, I won't wear half chaps with jeans, so they go hand in hand! Another thing is that jeans are slippery on leather and when you get a good pair of knee patch or full seat breeches, you are IN the saddle for good. No sliding or slipping whatsoever. Breeches are a life saver.
I also forgot to add that denim doesn't stretch nearly as much as the material they use for breeches, so with breeches you have a lot more mobility. Ok, I'm done now. I sound like a saleswoman. Haha