I think you bring up a couple of great points. It can be hard to escape the sort of lock-in effect if you've got a larger app on Heroku. For instance, I'm guessing it would be difficult for RapGenius to move somewhere else at this point. I think you also touch on the key argument for Heroku which, as far as I can tell, is "If the amount you pay is less than few hours of your time, why not?"

Thanks for the amendment to the comparison. In a sense, an ORM can still have ongoing "costs" as it tends to run a little slower and require more processing, so it might require additional hardware eventually as the scale goes up. Maybe I'm belaboring the analogy too much at that point.