On the other hand, bad virus updates have a habit of purging critical system files, leaving machines unbootable. They have the power to be extraordinarily destructive; good luck getting a browser to do that.
With that said though, except for high priority zero-days, we would actually roll ours up with our other patches so we can test them all together. There's only been a couple of times where we only did a smoke test and deploy def updates -- but we always do some testing on them. Always.