The advantage with using a harness and crayon is that you don't need to catch your ram so often in order to re-apply the raddle paint, which you might have to do every other day. You still have to check your ram's harness frequently to make sure it isn't rubbing, but this is much easier and less time consuming.
And harnesses work better on the larger breeds, just because they have more room around their brisket etc so the harness is less likely to cut into them.