I think you gain more value from a mage or warlock.
Shadow priest gets Shadow Weaving that increase shadow damage by 3% per stack, up to 15%. Most things will be dead before you apply it fully so very small damage increase for warlock.
Shadow priests have a lot of mana issues. They also don't have any AoE (except for holy nova if they spec into it...)

Shadow priest is basically a pvp class and they boosts shadow damage in raid since fights are long there.

Mage have channeled and instant aoe.
Warlock have only channeled aoe but can stun with pyroclasm (might trigger DR too fast when stacked, idk, think 26% stun chance per second per warlock).

Stacking mages can be good for more frostbite procs for more shatter. Although warlock and pala might break it.

I think having 2 warlocks you could use both curse of shadows and curse of elements, to boost damage of both warlocks and mage. Idk if that would be useful.

All 3 have silence.
Priest and warlock have dispel magic.

Warlock have dots and death coil which is useful for pvp but you probably won't use it for dungeons.

I would go for another mage for a PVE team and warlock for a PVP team, not sure if it is the optimal but for me it feels like it.

I intend to go druid tank (then in pvp I intend to use it as 2nd healer), holy priest, 2 warlock, 1 mage. 1 tauren+4 undead.