Well, currently UMMu in the hab module would be moot because there is no airlock in it: it provides living space for the crew and opens directly into the main cabin.
Also, while the user could technically remove the Crew Habitat Module (CHM) from slot 1 in the bay as you can any other payload, that would leave the crew with zero living space for food, water, bathroom, etc., so realistically you could only remove the CHM and carry other payload in that slot for short flights. Of course, there's nothing that will prevent the pilot from removing the CHM permanently if that's what he wants to do. By default, however, all the default Ravanstar scenarios will have with the CHM latched into slot 1.
Well suppose you wanted to use it for crew transfers while your docking ports were docked to other things (or broken
Doug and Steve!!