I'm hoping the vanity system extends to DoH/L classes.
Having played PvP in a MMO with no vanity restrictions in that mode all I can say if you haven't been able to get an idea of what basic role your enemy is by their sheer mannerisms within the first 2 seconds you deserve to lose. Whether they charge you (melee dps/tank), keep their distance to channel/open fire (ranged dps), keeping distance and a little too focused on their team (healer), etc.
Not to mention are you saying you would be able to positively instantly identify a class by any possible stat based gear combination that class might wear yet would be completely helpless to identify them by all their other tells? And what of shared gear set designs, such as Darklight?
I consider the need for gear appearance restrictions to be able to PvP a weakness undeserving to be catered to, much less forced upon the entire population.