Good question.

Given that this limitation is easily evaded, as a couple of comments have pointed out, I agree that it is a drawback as suggested in teh explanation, because it is simply an inconvenience which provides nothing useful. However, if it were properly enforced it might actually be useful - I'm not sure whether it would or not, because I really know pretty well nothing about central Server Management Groups, but I thinnk I can imagine a case where it might be desirable to give people some privileges on the servers in a group but not have those privileges extend to the central server; of course I could be wrong in imagining that.