Do any of our Ars Digitans know?
The case you describe is just about the only use for these "foo-admin" etc privileges I've heard. But as you say, it's really not sufficient reason if it imposes a performance penalty, and I know for certain it does. It would just be a convenience to subsite admins, that's all, and there are other ways it could be implemented.
If we only had a handful of privileges in the system, it would make the generalized permissions admin UI a lot more usable, too. Right now you're presented with a whole list of "foo-admin", "bar-admin" etc privileges you can map to a given object even though they don't have any meaning in most cases (elsewhere I've discussed possibly adding a mechanism to restrict the object types a particular privilege can be assigned to, but if we cut down the privileges in the system to a rational set this might not be necessary.)