There's been a lot of discussion on this thread on why we would support this; i.e. what the use case is.
I think it would be educational to hear more about why we wouldn't; i.e. what are the problems it causes.
At this point we have on the pro side IMHO kinda abstract use cases (I'd love to hear an actual concrete example where declaring subsystems wouldn't suffice) while on the con side it's unclear.