We can only omit this if we do a more global change to require that all content handlers supporting multiple formats auto-detect the format instead of having it be passed in. For all existing code that I'm aware of this seems certainly doable.
Topic on Talk:Multi-Content Revisions/Database Schema
Since most handlers only support a single format in the first place, this should be trivial. And it would save us quite a bit of work, and it would reduce the complexity of the schema somewhat.