Ah yes, it was actually Toph who "perceived" the impurities in the metal and used that to bend it. Platinum is used in Korra to counteract metalbending. While it may not be as tough as alloy or impure metal, it's a hell of a lot tougher than wood, and it would be used on people that are under watch most of the time anyway. It would give them the "gift" of freedom of movement, though.