Mages and Warlocks are similar classes.
They offer a lot of utility in general.
Either and both are very welcome, in almost any caster or mixed group.

Mages:
- Portals for faster/easier travel.
- Frost... replenishment for casters.
- Summoned Food (+Health) & Water (+Mana).
- Sheep vs humanoids/beasts.
- Heroism/Bloodlust equivalent: Temporal Acceleration?
- Easy to macro, strong caster DPS.

Warlocks:
- Summoning toons, to anywhere.
- Destro... replenishment for casters.
- Healthstone/Soulstones.
- Seduce vs humanoids, Banish vs demons/elementals, Fear.
- Variable pet buffs.
- Water Breathing, Curse of the Elements (spell power).
- Ranged AoE stun (Destro).
- Easy to macro, strong caster DPS.