This is a role playing game! Obviously guards aren't smart enough nor skilled to defeat us in a 1v1, but they are smart enough to tell whether or not we are fake or real using this trinket? It would be nice to fool guards and go undercover in cities.
In movies they always have their "spy" go into castles dressed as one of their own, and the guards always get fooled. Why can't this happen in WoW?
The only negative affect for the role playing aspect, is that the Orgrimmar orc guards may smell human flesh, like they do in Lord of the Rings.