The drawback to a healer on a PvP team, is that they're focused.
So you need a degree of survivability.

Less so in world/battleground play, where things are hectic.
And more so, in arena where it is five vs five.
But still, the healer (or different class) will get extra attention.

A Paladin as your healer can be a good choice, because you have three basic cooldowns.
Bubble is a survivability cooldown and Devotion Aura (previously Aura Mastery) will let you cast without interruptions.
All other class cooldowns go onto the "big button", which massively increases your healing through-put.
I also put the healer trinket on a separate keybind from the others, which has helped a lot.

If your characters are Destro Locks, they'll be standing and casting.
Which means a healer who stands and heals will work.
If you spec into Affliction and stay on the move, a more mobile healer (Priest/Druid) might be a better choice.



The flip side is, if you can survive without a healer, then you have a lot more DPS which means you need healing less.
The caveat is, can you survive without the healer?

My 5x Ferals had enough healing through Leader of the Pack and/or 5x Lifebloom/Rejuvenation.
They also had Predatory Swiftness, approximately once every ten seconds of combat against a single target.
The team did not need a dedicated healer for questing or battlegrounds; I don't imagine it would have worked that well in arena though.
By having five DPS, they were able to defeat quest mobs in Cataclysm that the 4x DK/1x Paladin team could not defeat; the Baron Geddon, in Mount Hyjal, was the only boss they couldn't "kill" while ignoring quest items that weaken the mob.
Conversely, the 4x DK/1x Paladin died on three other mobs that the Druids had no issue with.

So what survival, or self healing options do 5x Warlocks have?
Tier 1: Harvest Life (AoE Drain Life)
Tier 2: Mortal Coil x5 (15% heal, 45 second reuse) or a mixture of AoE Fear and AoE Stun.
Tier 3: Soul Link (Half of damage taken goes to the pet) or Dark Bargain (8 seconds of damage immunity, but then take 50% of what you would have taken over 8 seconds; on a 3 minute cooldown)
Tier 4: Unbound Will (same as your PvP trinket, on a 1 minute cooldown)... a 1 min PvP trinket, plus your 2 min PvP trinket...
Tier 5: Stronger Pets or an extra Pet for burst or no Pet (with 2% Regen every 5 seconds, and some spells are stronger).
Tier 6: Archimonde's Vengeance (opponent within 40 yards takes 125% of the AoE damage your team takes), which could kill someone who AoE's your team.
Glyph of Demon Training w/ Felhunter = Heals you, whenever your Felhunter devours a magic effect.
Glyph of Siphon Life = You heal 20% of your Corruption damage.
Healthstone = Heal
Soulstone = B.Rez

Is it enough survival/healing, to outlast opponents with your 25% increased DPS (5/4 = 125% of 4/4 =100%).