The 9th is so easy, so "all there in front of you" that it tempts you to overswing, to get yourself out of position, to make a 4-5-6 when 1-2-3 is possible. It's briliant in its simplicity. Ok that might be a stretch. But I really do like the hole, as is, for where it sits on the course and in the round. Taken alone I agree it doesn't have much architectural interest.
From Doak's original ranking definitions a 10 was "even if you missed one hole you'd miss something interesting". I think he's recanted on that a bit, but under that strict definition maybe TOC isn't a 10, but I sure think it is given where the ninth hole falls in the round, etc. The sum is worth well more than the parts.