Casual arenas aka not ranked are something needed... when people want some PvP and you ask them to run 20 minutes in Cyrrodil only to get killed/ganked and run again for 1 minute of action they will go PvP in another game... and you slowly lose them.
What you are talking about is RANKED Arenas and there i maybe agree but you should keep in mind THIS IS NOT [censored] WOW, the classes here have some unique skills but all have access to the Weapon/Guild/etc. skill lines, so the only balance they must ever do is with class skills.
Either way to appeal to the public they must offer as much as they can, i am aware there is a vocal community that wants only Open World PvP and fear arenas might take people from that, but what they are essentially saying by that is that some people would prefer Arenas to AvAvA... so why not give them the option?
You need people to find that everything they want to do is available in your game or they will log out and try other games... however if all they need is available there is a less chance of that. If you want this game to survive(keep in mind the fate of the other subscription MMO`s) you will never stand in the way of them adding an optional feature already present in most of the other MMO`s.