I've always thought that in the case of a single major defect, you should decide what the book would grade otherwise, THEN detract for the main defect... i.e, if your book would otherwise be a 9.8, than a detached CF at the bottom staple would bring it down a couple of grades, to a 9.4... same book, COVER detached at the bottom staple would bring it down to a 6.0, since any cover defect is FAR more damaging to the overall aspect of a given book than the same damage to a cf...
I think that what CGC does, from what I have observed, is something similar to this line of reasoning, taking it as their particular mission in this hobby to grade each book on its' own merits, irregardless of the grade given for similar damage to other books... to some this is indicative of inconsistency, but to me it is simply a different way to grade, and one that makes a certain kind of sense...