I’m not saying I agree that we SHOULD have a glyph to stay perma-visage even in combat, but there is reason for it, in-game.
In combat, Wrathion, Ebyssian, Sabellian, Nozdormu, Kalegos, Chromie, Alexstrasza, and virtually every other dragon that has ever been depicted in combat, has done so in their Visage form. Only when facing something as large as another dragon, or when being a dragon-sized creature was advantageous, for instance to intimidate their opponents, did they turn into a dragon.
We’re mortalized dragons. Our visage changes should also be a choice, not a requirement.