Have you actually asked CGC that they do downgrade for chipping if a book is not in lower grades? I do agree that lower graded books already have enough defects on them that mc does not really do justice to the final grade. On the other hand, mid grade to mid/upper (6.0-8.0ish) I do not see a drop in grade at all if mc is present. I think I also read years back about the issue of mc that CGC grades the book as if the mc is not there on a chipped book.
Yes, it is not precisely what I meant, just an example that a gap is in fact present and that it is more significant now than it was before.