The reason it is disabled during pvp is probably so that people can't hide their PvP armor with PvE armor. A common tactic in other mmos that don't hide vanity options was to change all their PvP gear to look like PvE gear so people would be like lets go for the guy using all that PvE gear only to have a hard time killing that player because he has tons of def from player attacks from his PvP gear.