Even with revisions turned off for a given content type, the field_revision_* tables for the fields each contain a single record that matches the corresponding record in the corresponding field_data_* table.
I've always been aware of this, and previously, I've never had a reason to question it. However, now I am dealing with an extremely large database, and almost 50% of the database size is made up of this duplicated data in the revisions tables.
Why do we duplicate all the field data in the revision tables when revisions are disabled for a content type?