Decision - Should You Play Coordinated Attack?
TL; DR:Gitz and Chainrasps should definitely consider it. Guard and Skaven might pick it up, and the other warbands should probably pass.
Obviously, Coordinated Attack favors those warbands with a lot of models over those with just a few. None of the Sigmarite warbands are likely to benefit very much from playing Coordinated, and the benefit for Orks, Dwarves, or Fiends would be pretty limited. Eyes of the Nine should also probably steer clear, as swarming around an opponent isn't really their style.
The two 5-model warbands can both benefit from Coordinated Attack, but Skritch's fuzzy friends probably reap more reward from it than Garrek's Reavers. Unfortunately for the Reavers the kill-and-die indiscriminately playstyle rewarded by their inspiration condition doesn't mesh well with this particular upgrade. Skaven, on the other hand, can almost always generate a few supporters to back up whoever has Coordinated Attack, and the three scrub models gain a significant improvement from the attack itself.
The 7-model warbands benefit even more from Coordinated Attack, thanks to the mobility actions of their leaders; that's right, Varclav is the real leader. Petitioners and Chainrasps alike can get an upgrade to either accuracy or damage (and to both, in at least one case) from the base attack, though The Ever-Hanged and the three named skeleton minions won't. As we'll see shortly, however, the real power of this upgrade comes from the better-than-linear enhancement it receives from supporting units.
Finally, the Gitz are probably the best warband to take advantage of Coordinated Attack. Not only do they have a plethora of models on the field to surround their enemies with, Scurry allows them to move several supporters into position at once and attack in a single activation. Melee heavier models like Drizgit and Zarbag probably won't equip this upgrade (and the squigs can't), but Prog and the three archers all benefit from Coordinated Attack's increased damage versus their base attacks.
Factor: Compounding Accuracy
The real power of Coordinated Attack comes with the way that it scales with supporting models. Let's take a look at a couple of examples.
At base, Coordinated Attack does 1.11 damage per unblocked use. However, 2-swords to hit isn't great, and that damage output drops significantly when we include defenses. Against a 1-shield defense, the attack action from this upgrade will only do an average of 0.8 damage per use at base. Against 2-dodge defenses (an increasingly common defense stat with the release of Nightvault), this number drops to 0.70 per use, and against 2-shields it sinks to a miserable 0.6 damage per use.
However, when we start adding supporting units to the equation, we start to see a significant uptick in usefulness. With 1 unit providing support (assuming none for the defender), we see an increase to 1.4 damage per use against 1-shield defenses, 1.22 damage per use against 2-dodge defenses, and 1.08 per use against 2-shield defenses. These numbers increase further (to 1.76, 1.6, and 1.52, respectively) when we add a second supporter.
The compounding effect of Coordinated Attack generally ends there, but it continues to increase in accuracy and average damage output per use with each supporting fighter beyond the 2nd, although at a reduced rate of marginal gain. Even if the marginal gain is relatively small, there is definitely something to be said for hucking 7 dice, all of which succeed 66% of the time, at your opponent's model once you have them completely surrounded.
In situations where your opponent also has supporters, these benefits still accrue, although the curve gets pushed to the right. Initial supporters on your side will provide extra dice only, and the compounding effect won't begin until you outnumber the bad guys. Nonetheless, this provides an accuracy bonus that large warbands can take advantage of even in situations where they wouldn't normally gain any benefit from supporting fighters.
Factor: Adjunct Cards
A less obvious benefit to using Coordinated Attack is that it enables some commonly used (and quite a few less-commonly used) cards, making them either easier to use or more effective. Trap and Pit Trap, for example become very reliable off a 2-supporter attack with Coordinated Attack - as a hit or a tie is likely in over 80% of cases against all but the strongest of base defense stats.
In terms of Objectives, Coordinated Attack really shines in enabling those objectives that require you to hit with a certain number of attacks in a round, such as Drag them Down or Let the Blood Flow. Obviously, with a sufficient number of supporting fighters, you're likely to hit with Coordinated Attack, moving you partially toward that goal. Less obviously, setting up a high-accuracy situation for your fighter with Coordinated Attack will also likely lead to a situation where your other models are also benefiting from support against the same target; an ideal situation for firing off several successful attacks.
Similarly, Coordinated Attack is great at getting Unstoppable to score, if you manage to catch an enemy model while they are on guard.
Alternate Path: Mutating MaulWith the Awakened Weapon and Helpful Whispers relegated to the restricted list, accuracy enhancing upgrades became much harder to come by (the main exception being Potion of Rage, but it's really more of a ploy that you have to pay for). Mutating Maul provides an accuracy boost against models with a shield-based defense, while still providing a benefit - if a small one - against dodge-based models as well in the form of knockback 1. Since both attacks do 2 damage and have a range of one hex, they are fairly easy to compare.
How does Mutating Maul stack up to Coordinated Attack? That depends on who you're trying to murder. Against dodge-based models, Mutating Maul will hit at-base about 12-15% more often, depending on the number of defense dice. However once supporters start showing up, Coordinated Attack begins to gain significant amounts of accuracy over Mutating Maul. With just a single supporter, Coordinated Attack becomes more accurate against all dodge based defenses than Maul by an average of 3%. With two supporters, that number increases to 8-17% depending on how many dice your opponent is rolling to defend. After two supporters, Coordinated Attack will continue to improve, while Mutated Maul will stagnate, furthering the gap between the two.
Against shield-based defenses, Mutating Maul starts off the comparison even better, hitting just over 25% of the time more than Coordinated Attack at-base. Versus these more hardy defenders, Coordinated Attack will continue to lag behind by around 7% in accuracy even with one supporter. However, with two or more supporters, Coordinated Attack becomes more accurate, even after factoring in cleave.
In the end, Coordinated Attack is going to give you a greater advantage than Mutating Maul any time you are able to conjure up at least 2 supporters (or 1 supporter against dodge-based armies).