I think Locks are a decent team.
More so for battlegrounds, than for arena; not saying you cannot do arena, but would be surprised (surprise me) if they got a high rating.

Warlocks can have an insane amount of hit points.
Basically, they're the only class that with equal gear is not going to be one-shot by 4x Enhance with Ascendance.
Not saying you won't take one out, in the Ascendance window... but they have the health, that someone could react and give them a BOP or whatever rather than one-shot them.

In Tier 2, you could mix Shadowfury (ranged AoE stun), Howl of Terror (AoE Fear) and Mortal Coil (Insta-cast Fear that heals the lock for 15%).
In Tier 4, Soul Link or Dark Bargain.
In Tier 5, Unbound Will, which is a one minute PvP Trinket... so you could go 2x DPS trinkets or if they don't share a re-use lockout, a one-minute trinket effect and the normal two-minute trinket.
In Tier 6, either increase your AoE radius by 500% or an enemy takes 25% of the damage you do for 8 seconds... could quickly kill someone if your team was subject to cleave/spell-cleave effects... I like a huge radius on Seed of Corruption AoE too.

You wouldn't necessarily want Demon Form (almost anyway) constantly, but with Demon Hunting (major glyph) you get a persistent Apophesis... which makes you that much harder to be killed.

I like all three specs.
Again, more so for battlegrounds than for arena.
Affliction is dots on the run, with a lot of decent self heals... could combine with a Priest or Druid healer.
Demo has the pets.
Destro has Chaos Bolt and the interaction with Havoc.

One of my teams will be 6-7x Warlocks, with 3-4x Resto Druids.. )