Well, it's not just hosting though. Typical hosting you cannot just boot down and then boot up a server with 8x the ram or disk space. That's just one thing.
>> "...and pay by the hour"
If you have money to burn.
I can see the case for this sort of scalability if your traffic is wildly unpredictable - eg 1 visit one day, 100,000 visits the next - but I think that's rare.