Warlock do have a use in group ?Follow

WArlock and mage have different uses and get their damage in different ways. A warlock's damage over time spells are are potent, sure, but they're not front-loaded. By definition they take time to kill. A mage, on the other hand, deals his damage in large bursts. This means that the group can burn through individual mobs more quickly, which not only speeds things up on small pulss, but can be a key to evening the odds when things go wrong. In fact, when the group is killing quickly, a mage is MORE mana-efficient than a warlock (because the DoTs don't last their full duration).

This leads us to another big key to the mage: Polymorph. Polymorph affects humanoids, which constitute at least 40% of dungeon mobs, whereas a warlock's Banish only works on demons and elementals. Polymorph is also ranged, which gives it a big edge over the warlock's other CC option, the Succubus' Seduce. A well-timed polymorph on an add or patrol can save a wipe in the blink of an eye. There are probably dungeons where both the warlock's CC options come into play, but Polymorph is much better than either Seduce or Banish individually.

Mages can summon food and drink. They don't like being known for this ability, but they can do it. It reduces downtime for everyone, which makes everyone happy.

Nothing I can think of on the scale of poly, fear, and sap.

Warlocks are valued for several things in a group.
Lets look at what a Warlock brings to the table when you start really grouping.. say at 22.

Pets/damage- duh. Pet can off tank pretty well.

Curses- Damage caused by the target is reduced by 10 for 2 min. Only one Curse per Warlock can be active on any one target.

Rain of Fire (AE)- Calls down a fiery rain to burn enemies in the area of effect for 168 damage over 8 sec.

Ritual of Summoning- Begins a ritual that summons the targeted group member. Requires the caster and 2 additional people to complete the ritual. In order to participate, all players must right-click the portal and not move until the ritual is complete.

Create Soulstone- Creates a Minor Soulstone. The Soulstone can be used to store one target's soul. If the target dies while his soul is stored, he will be able to resurrect with 400 health and 700 mana.

Create Healthstone- Creates a Lesser Healthstone that can be used to instantly restore restore 250 health

Eye of Kilrog- Summons an Eye of Kilrogg and binds your vision to it. The eye is stealthy and quick, but very fragile.

So the Warlock can offer instant heal items, ressurect items, scouting eyeballs, summon party members, AE damage, single target damage, off tank with pet, fear, and massively debuff a monsters attack damage..

This isnt even taking into account imp buffs if you pick an imp pet instead... group damage shield, group HP buff, etc.

Doesn't seem too useless eh?

Mage/Warlock combos work awesome in instance groups. The mage can frost nova a group of mobs and the warlock then fire rains while the mage is also blizarding them. Massive AOE damage, the warlock provides security because they can soulstone the paladin/priest/druid (whoever the primary rezzer is) in case of a wipe. Mages do pull ahead in the crowd control area, but warlocks have more overall utility. Neither is "better" for a group, both rock, and when combined its awesome.