I run 5 locks and they are superb team in pvp with some decent gear.

Felguards almost other warlocks, mages and non disk priests.
10 instacast dots and seed of corrpution can decimate enemy raids in seconds.

After you get fel armor and lolcoil you almost will not need a dedicated healer because fel armor + health stone + lolcoil + drain life will top you from 5% to 100% in a couple of globals.