I imagine Heroku would say the same, and thats fine. We deploy a bunch of Rails apps, backed by Postgres, via Heroku and most of the time what we're doing fits neatly into the 12 factor definition.

However what happens if you're doing something that doesn't really fit that definition. For instance we have a Rails app with some background workers doing data processing. I would very much like to have these workers just dump to disk, so I can take a big chunk of data and move it into Postgres at once. But I can't do that with Heroku.

So basically this is something that Heroku isn't designed to do, but its also something I would rather not need to go one level of abstraction deeper, to AWS directly for instance, in order to do. And its also something that every single one of their competitors offers.