I guess you're suggesting I leave the machines on and hope they all update themselves in the background.

I don't think that would really work. Not all the changes I make to machines before a party are things that they'd do automatically if just left to sit. E.g. I usually install some new games some people suggested, or download the latest nvidia driver directly from the web site (where they are available before Windows Update gets them), or remove games we aren't playing anymore to free up space (or because they are constantly downloading enormous updates wasting banwidth), etc.

Also, I don't actually leave the machines running outside of parties, and updates don't just all happen immediately when you turn the machine on... I'd have to start them up a few days in advance.