I personally think that a lot of it depends on what you are going to do with the horse. If you are just looking for a pleasure or trail mount, and he passes the prepurchase exam, then I wouldn't get x-rays. Now if you were getting a horse you are planning on jumping, or doing high impact sports with, reining, barrel racing, that sort of thing, I would do x-rays even if he passes the prepurchase exam. Age also plays a role as well. If he's older, or young, and has been used hard, I'd do x-rays even if all you want to do is trail/pleasure riding. I've seen people advertising jumping prospects in my area, that are 4 and 5, and already consistently jumping 3+ feet and showing, and I would be concerned what type of damage, or abnormal bone growth the x-rays for those horses would show.