Guard glyph can be useful not for the 10% increase (because that's a pitiful amount really) but because it blocks only magic damage and BrMs have almost no defense against magic. For example, on heroic Siegecrafter, he hits like an absolute truck during Protective Frenzy. If you Guard there, you'll block a couple of his swings. However, if he uses Electrostatic Charge during this time and is swinging like a madman and your Guard was already eaten up, you can get gibbed very quickly. If you have the glyph of Guard though, the normal swings don't eat through it and you eliminate the spike in damage from the Charge. It's more damage taken overall, but no one actually cares about overall damage taken as a tank. The important part is making sure you don't take 1 million damage in 0.5 seconds.