Demonology spec for a warlock is less maintenance then Affliction.
By that I mean, with fewer keystrokes you're closer to your optimal effectiveness.

But as suggested above me, Affliction will level faster then anything else.
Everyone has instant Corruption now. Combine this with instant Agony, instant Siphon Life.

With a single warlock farming affliction spec, I had a castsequence macro for Corruption/Agony/Siphon.
Never stopping my movement, just targeted a mob and clicked the button three times.
And kept running along. Made the deal with my roommate to fun behind me and loot the corpses.
He could not keep up, and the mobs were only 1-2 levels below me at the time.
I was running warlock (me) and warrior (him) in the party, with my holy priest out of it.
The only thing the priest did was heal my lifetaps when I was low on mana.

Running four locks would be pretty sick.
You'd have UA, Haunt, Agony, Corruption, Siphon Life (x4) along with 4x single fear.
You have 4 warlock and 1 priest AoE fear.
For tougher mobs, two dots each... Corruption/Agony and SW:Pain/Devouring Plague.
For most mobs, one dot from each toon.
Mana won't be much of an issue for your team; one or two dots each is very little.

I like the idea of 4x Felguards in an instance.
The Improved Health Funnel talent, in conjunction with the Health Funnel Glyph is OP...
Even if the solo-lock Health Funnel is very much weaker then the solo-hunter Mend Pet mechanic.
A guy posted on these forums how he instanced entirely with 4x Demo Lock team, 61-66 in Ramparts/Furnace/S.Pens/M.Tombs.
It's a pretty sick combination... and that was 4-box without a healer.