@Networkz: Yeah, you're not going to out-"gain" a semi-competant shadow priest, but you will out-"gain" some classes and terribly geared players. I will have an official "When it's appropriate" list up shortly.
Note that this would only apply to Destruction warlocks who can get the +2 Immolate tick as a result of stacking. It is ~never~ appropriate for a Demo or Affl Lock to do this. The gain is simply non-existent in these two cases.
---------- Post added 2011-02-10 at 11:06 AM ----------
The Warlock Circle Jerk, and when its appropriate:
First off, it is never appropriate for an Affliction or Demonology Warlock to receive it from another Warlock. The gains simply are not there. The only thing you gain in this situation is 3% Haste for an approximate 600 dps. That doesn't even register on the gains any other class or healer would have. The only situation this might work is 2 Paladin Healers, 2 Death Knight Tanks, 4 Warriors and 2 Warlocks, but if you joined this raid then God have mercy on your soul shards.
The conditions for this situation are very precise:
Two Destruction warlocks who, with 1 Dark Intent each, sit at exactly 26.2233% Haste raid buffed. This means that with a second Dark Intent they will be at 30.01% for the extra Immolate tick and Conflagrate damage, with zero extra haste wasted due to passing this "soft cap". The .01 is important because sometimes Blizzard rounds funny, and we want to play it safe.
Two Destruction Warlocks + Any Two Other Raiders
26.2233% / 3% DI / 5% Raid Aura = 16.71% from gear = 2140 Rating.
First, lets run the profile through SimCraft, adjusted to have 2140 Haste Rating.
No DI: 23178
1 DI: 24272
Copy paste of SimCraft stats for Haste: Spell Haste 26.22% Raid Buffed, 2140 Gear Rating
Now lets give them a second DI (increase spell haste rating to 2590 - the 30.01% haste cap.)
New DPS: 25294
Gain: 1022 DPS.
That's fairly substantial, lets add it all up:
Destruction Warlock 1 Gain from First DI: 1094
Destruction Warlock 1 Gain from Second DI: 1022 (Haste Only)
Destruction Warlock 2 Gain from First DI: 1094
Destruction Warlock 2 Gain from Second DI: 1022 (Haste Only)
Raid DPS gain: 4232
Look at the Destruction ranking charts for your particular guild.
Take the two highest ranked classes you have and add them together.
Is the total above 4232?
YES: Circle jerking is bad
NO: Enjoy the self love
The question will remain: does this apply to your guild and your players? Most of the time, yes. Players of similar skill levels tend to play together, but not always. However, I would like to make one thing very clear: a badly played Shadow Priest will still equal if not out-perform Dual Destruction. This is because they are so far ahead of everyone else in terms of the 9% buff. If your Priest is so bad that Dual Destruction is better, then I encourage you to rethink inviting this person to your raid in the first place.
---------- Post added 2011-02-10 at 11:08 AM ----------
One thing to note: I used 2590 Haste Rating instead of 2588 because SimCraft did something very peculiar - the difference of those two points registered a gap of 400 dps.