The reason for using them is creating a workflow, where in the early conceptual stage you can model the building form with In-Place mass(es), automatically create rooms based on those masses (i did it up to that point) and pass the names of Mass Subcategories as e.g. the Room Departments’ names (this step is a problem). By doing that I can automatically assign to the rooms the same colors as the to the Mass Subcategories (assuming that I defined my color schemes prior to that). I do not think that I can achieve such workflow with anything else than In-Place Masses.
Is it really impossible to get those subcategory names? I found a solution to this problem (or so it seems) proposed by Jeremy Tammik here: http://thebuildingcoder.typepad.com/blog/2012/11/geometry-object-subcategory.html - I just cannot find a way to use it in Dynamo environment with Python. Any thoughts?