I've also wanted a flatter modeling system for a long time, where parts and assemblies weren't different, for just the same reason. Plus, it would make adding material to assemblies easier. Real solid welds, for example.
For example: ready-to-use casting and someone comes along later and wants to add an insert or a sticker.
On drawings I'd just add a new dash number and views showing the assembly of the items; I don't see a need to remove or replace existing views of the base component. It's the higher assemblies where this becomes a problem. Not only does it require replacing and rerouting, but all the next higher assembly drawings need a close inspection for leaders that no longer point where they should and repeat regions that need to be repaired.
I'd toyed with the idea that there would always be an assembly for each part, but that screws up the PDM side and makes a lot of other activities harder. For certain someone is bound to rename the part and not the stand-by assembly, leading to a lengthy search for which name is correct.