Q: Is it considered MB to wear an animal's entire wardrobe on your head?
A: Well, it's pretty magnificent to be heir to the throne of the fading empire that gave us the Magna Carta and golf, and wearing hats like the one Prince William was wearing this weekend is part of the job description.
As for anyone else? William's bearskin hat is certainly characterized by a senseless lack of utility, and scores high on archaism, organic materials, and Anglophilia as well. But its primary historical purpose — to make a soldier look bigger and more imposing in battle — violates the principle of understatement and essentially establishes the garment as elevator shoes for your head.
As you allude to, the standard hat of the British Foot Gaurds is made out of an entire bearskin. It weighs 1.5 lbs. and, most consequentially, stands 18.5 inches high. Getting in and out of limos and taxis would be a huge hassle while wearing one of these things, so until horseback reemerges as the predominant form of travel, we say "pass."