I like the idea of that! A bit like independent worksheets for 3d stuff.
I could see it being quite useful for putting together little 3d details or junctions, like detailed stair components for example, without cluttering up the floorplan or keeping them in a separate file.
I don't like the idea of having 'fake' storeys or an area to the side of the main model to keep bits on. Not very BIM-like when you think about it, so a separate area would reinforce that concept.
The ability to hotlink those into the main model could be a very powerful addition.
Where do you stop? If you could give each 3d workspace a predefined label so it knows what it is, eg a window or a door, the 'module' on the floorplan could behave as such, and inherit wallhole cutting properties, and skin turning behavior. That is probably heading into in the realms of fantasy though!