That's what WE said. I'll bet you don't have a problem telling the difference between a 69, a 70, or a 68 for that matter - either. Right?
To my mind, the lower-graded face determines the grade. But, if you see one face as MS and the other as circulated, it's obvious you need to look more closely at the coin. We, unlike the TPG's, have the luxury of studying and contemplating the coin at length. There is much to learn if you can but devote more than superficial attention to your object of study.
But I see it as a way to test the soundness and strength of the grading criteria itself. I try to "break" the system, sort of speak. Maybe a different grading scale will fare better. And, in the inevitable case that even one or two rare possibilities does come across the grader's hands, what would they do? Make an exception and go with the convention?
Meh... I'm not bothered by questioning authority, mostly because I'm simply curious as opposed to trying to pick a fight or whatever lol. And, if the Sheldon scale as it is today is, for all intents and purposes, sound, then I'm sure hardcore advocates would / should welcome the challenge. (I'm not advocating one system or another; I'm indifferent with the various grading scales that are used for all collectibles, not just numismatics - I'm sure they each have their applicable strengths and weaknesses).