In WoW 5.0, activating Shield Wall no longer requires you to have a Shield.
I've been saying it for a while now. They are making WoW a better game by making these flavour sacrifices and nonsensical spell effects. In terms of gameplay, it's poor design to require players to create a macro just to access a baseline ability.
If you want flavour, I recommend Skyrim as a great game that isn't afraid to bog the player down with "realism".
[Not to disparage Skyrim. It's an excellent, excellent game.]