Hourly billing does present a different usage pattern though - for one thing, when people spin up servers on-demand it usually means they want to use a lot of a particular resource right then, affecting how hard you can oversell. Also if people can spin up new instances for a couple of hours/days to cover peak usage then maybe they won't upgrade to a higher plan as soon.
Of course lots of people are just going to spin up instances and keep them running permanently (because automatically deploying instances is harder than just setting them up manually once), and some number of people are going to sign up because if they cancel it after a day or two they're only out a couple of bucks, so the marketing benefit may outweigh the downsides.