Sometimes you pressure the perimeter so that you can prevent easy ball reversals and easy passes to the wing. Easy ball reversals and swing passes to the wing make it much easier for the guards to feed the post. Depending on the personnel you have, you might choose to defend a team with bigs differently than just sitting in a sagging zone. It would probably be an interesting question to ask Sherri, but just because she didn't sit in a sagging zone like you would expect doesn't mean she didn't have excellent reasons for doing so.
If you go back and look at who was successful against OU when we had Courtney Paris, it was Texas A&M. They certainly didn't beat us with a sagging zone. They pressured the heck out of our guards & made it very difficult for the guards to get Courtney the ball. The teams that sagged on Courtney still gave up 20 points and 10 boards, and our guards killed them, too.
I think Sherri had confidence in her guards' ability to defend the perimeter and didn't want to make it easy on Texas. We doubled the post & pressured the guards to make it hard for the ball to ever get there. It looked like a pretty darned good defensive scheme to me.
Perhaps the coach with multiple Final Fours with all sorts of different personnel knows what she's doing over there on the bench.