Wouldn't this be good for shutting down unused VMs in order to save money when renting infrastructure? Most website can go "down" during the night and when a user tries to connect to it, it would take 0.2s extra, but after that it would stay on.
Now that I think of it, spiders/bots might activate it beforehand. You could use robots.txt, but not all spiders/bots seem to follow those "rules".
Heroku actually does this for their free instances. They "sleep" servers after iirc 30 minutes of inactivity, and they wake up again after the first request, which takes a few seconds longer as it boots up.
What does a website really do if it does nothing? What money does it consume? What else can you do with the machine it's on? Wouldn't moving VMs around accomplish the same goal?
Now that I think of it, spiders/bots might activate it beforehand. You could use robots.txt, but not all spiders/bots seem to follow those "rules".
Food for thought!