yes, the jeans allegedly prevent chafing.
sometimes the inside seam of the jeans is problematic/painful and chafing.
but, you will not be doing a lot (or much at all) leg work with theses horses.
you will be riding low in a western style saddle, with your legs forward, and heels down.
ya, you may get some rubbing on the inside of your knees.
in aruba, i ride in shorts.
in florida at home,i ride in shorts.
the legwork i do is mostly heel and there is very little rub on the leather.
now, if you were to be riding hard, at a gallop or a canter and loping a lot, then you might get rubs.
with walking, not so much at all.