To be able to select certain members and group them (not as GROUP tool) like an assembly. Then, if any change needs to be perform in a member of the assembly, you isolate the assembly, change one, and then all the copies update also.
I find this useful then designing trusses. Now if I have 20 copies of a truss code T1 I have to change a web or a chord in all of the 20 copies or more. This option would be much more efficient.
Came across you idea and just wanted to point out that you can have multiple section assignments that use the same section, so the capability is already there. Just assign sections you want to keep the same under its own section assignment.
Can you clarify if the above doesn't work / align with your idea?