Agree with TC. Guarding is something you should do automatically with a shield, because otherwise what exactly are you holding the shield for? Hell even with it just hanging on your arm, fighting random animals you would naturally block some hits just incidentally. It's flow breaking to have to completely stop to raise your arm about 6 inches (or worse, when guard goes down and you stop mid-sprint to lower your arm about six inches). With auto-attack coming, it's just natural for auto-guard to be in there as well.

I'm a tank in all games I play. In general, I've noticed the games with the best tanking mechanics focus more on holding hate rather than trying to keep yourself alive. The tank should be tough as a matter of course, the skill is supposed to come in keeping the enemy off of other party members. Keeping you alive? That's the healer's job, not yours. Not to say there should be no active damage mitigation abilities, that's just silly, but having to manually raise your shield arm is getting close to having to press a button to blink.