You beat me to it Eric ,I had the same concerns on its vunerability in rough going but for getting the brass snapped off rather than the tap coming open. I have just looked at the you-tube Vid of one in operation and agree that the tap is a weak point even though it has a two stage action to open it.I'd be a little worried about putting a drain valve like that on a tractor. Seems to me if your doing alot of heavy bush hogging or grading something might flop up and smack the valve causeing it to come open and loose all your oil.
If I am not in a hurry or the drain plug it hard to get at I use my vacuum oil extractor pushed down the dipstick tube when I change the oil.
Dave