Arcane is either Arcane Blast or Arcane Blast x4, Arcane Missiles.
Fire forces us to watch for procs.

Frost is Frost Bolt, and summon the pet on every cooldown.
Frost also gives replenishment, and with the snares/freeze chance is extremely nice for a caster/ranged team.
Not to mention the food/ports etc that a mage brings to a team.

I've respec'd my mage to frost, on my mixed team.
The druid is Feral, but I'm really looking at dual spec for Balance too.
Even in caster form (pretty pathetic with feral talents and gear), almost everything dies before closing to melee with 5x casters.