If it's lost enough compression and isn't firing on that cylinder, all that unburned fuel is washing the cylinder down and wearing it. I guess it would also matter why he's losing compression. If it's a valve it might not do anymore damage. If it's a headgasket or rings it could possibly hurt it more. Then, vibration of a dead cylinder comes to mind. Not sure how hard it is on a motor but it can't be good for it.
