You know how sometimes you hear a singer produce a note that's obviously insignificant to their range, but it's so powerfully produced or sounds so good that you remember it? What are some of the best of these notes you've heard? An example for me would be some of Michael Jackson's falsetto yelps.
If it were any other singer, we'd count it. I think we're just avoiding the painful scenario of a new user asking, "Frank is often considered the greatest singer of his era, if not of all time. What does his highest note sound like?" and having to show them that C5. But the more I hear it, the more I think it should be counted, painful as it is.