In the interest of full disclosure, I've done that. Just not paying attention.
You can also make a decent point that by the time there's one target left from an aoe pull, your dps doesn't actually matter all that much. It's got like 3% hp left and switching to single target at that point will kill it 0.000000000001 sec faster.
If random outcomes aren't acceptable to you, then don't play with random people.