The mages crank out some serious DPS, I am fond of my Mage in my 10box sidebar team once I went Arcane and once I got some serious gear for it. Until the gear I was seriously thinking of dropping it. Plus arcane potency is nice and hard to replace integrate in a lot of common boxing setups. I guess amp / dampen magic is nice too.

I don't see how multiple mages add any synergy, more than one is an opportunity missed.