I don't do any PvP with my team, it simply doesn't interested me.

As far as PvE there are some nice things about warlocks:
If you're in an instance and need to go back to town to sell/repair, you'll have a way to get summoned back.
3x soulstones + anhk means you'll almost always have wipe recovery (except when you forget to soulstone your healer, which I do constantly).
If you spec 0/41/30 your rotation is pretty uncomplicated.
Damage-wise they do pretty well.

I tried speccing affliction and it was a nightmare to be efficient. I could do it, but between tanking, healing and keeping track of dots it was a headache. If I let a dot or two fall off I'd lose track of what I was doing and it would cause problems. Nowadays with 0/41/30 it's a simple rotation that does decent damage on trash and bosses and it keeps me sane.

I'd suspect with totems and blessings your team would do pretty well PvE-wise.

Edit: (Sorry, didn't realize you were comparing this to a 4xshaman + tank group).

As far as a comparison to the tried and true 4xShaman + tank group I'd say the biggest thing you'd miss is Heroism/Bloodlust. Honestly, I think this is the biggest draw for this team. With this you can bully down bosses you'd normally be undergeared for. In my opinion this is the biggest reason that these teams are so successful.