My 4-lock + priest team is 3 felguards and a felhunter. Why? Rogues/druids and other squishies. Felguards do excellent damage in PVP against non-protection-spec classes, which makes them very good roaming DoTs that don't need LOS, have intercept stun and can purge buffs off them. Rogues and druids HATE my felguards. 8 DoTs plus felguards = dead whatever. Oh, sure, affliction is better dot damage but I find it less to my liking than demonology. Plus, I really like the Mana Feed talent where the demons get a percentage of mana when I lifetap. Keeps them cleaves and intercept stuns going.