Hah, thanks, Saelorn, and I was indeed thinking of that passage from HPMOR. And now I'm thinking of how Potter challenged the assumptions about Transfiguring part of an object... and what he'd try in Forgotten Realms.
Along the lines of TwoSix's post, there might be recreational uses of Polymorph, or as perhaps it should be more clearly named, Polymorph Into Beast. One could have an alternate identity, and a social life under that identity, which was furry as a tomcat; there might be venues in which polymorphed wizards and wildshaped druids meet and compare experiences, particularly at certain seasons of the year. I nominate Phil Foglio to research this, since there were related topics he never fully resolved in the "What's New" column in Dragon Magazine.
Back to more practical uses: Polymorph Into Beast seems to me an amazing utility spell. It subsumes Water Walk, Water Breathing, Spider Climb, Jump, etc., and most uses of Enhance Ability; you can carry tons as an elephant, dig as a badger, travel overland as a horse, sing as a nightingale, track as a bloodhound, see for miles as an eagle, pass through tiny spaces as a mouse, topple trees and build dams as a beaver, chuck wood as a woodchuck. You can't spellcast while flying, but if you can poly into Roc then you can carry the whole party. As a Giant Ape, you throw rocks like a living catapult. As a T-rex, you win the village's annual "who can eat the most hot dogs" competition... as long as you have a way to get them into your mouth, other than using your inadequate forelimbs.