The problem is, in my opinion, that they almost require that attention. A rogue unattended = much pain for your team. And a mage is even worse, because not only do they bring "jaw-dropping damage" (=P), but an incredible amount of control to their team. They cannot be left to do their bidding, imo. While both of these classes are tough first targets, I personally feel that you have to a least threaten them or they will just pull you apart over time.

BTW, that's all just speaking about general arenas, not specifically boxing arenas (although I'd assume the same is true).