I created an idea already for this and piping, but forgot that is is not just piping.
http://forums.autodesk.com/t5/revit-ideas/pipe-duct-demolition-keeps-system-designation-and-stays/id...
All elements should keep the connection to their original system, or somehow maintain the relationship to the original system designation.
Within electrical if you demolish something that element is removed from the panel. If you have multiple phases and show elements on the panels at an early phase, you can't do this. So you have to just work around the phasing for those elements. NOT ideal.
If this was handled a bit differently then we could still use the native Revit systems and the native Revit phasing tools.
If a wall can infil after a door is demolished, I'd think something could be done about the phasing on the MEP side of things.