It's another long cooldown
At its heart, Shadow Blades is just another pointless cooldown in our repertoire. While the extra combo point aspect is a nice touch, the fact is that it forces us into an Anticipation-based build. Gaining combo points in all twos or all threes doesn't help us when we our combo point bar only has five slots. With Shadow Blades active, you're going to gain perhaps five extra combo points during its short 12-second duration. It adds up to one extra Envenom or Eviscerate per use, not accounting for the 10 energy lost, every three minutes.
I feel the worst for assassination rogues, who are still stuck with Vendetta as their only major cooldown with the removal of Cold Blood in Mists. Vendetta is still the least cool cooldown of the bunch, with Adrenaline Rush and Shadow Dance actually being quite fun to use. It's just another cooldown that you'll mindlessly pop with all of the other ones. I was hoping that Shadow Blades would even the playing field, but it turns out that SB is actually worse for assassination than either of the other specs. Ouch.
Combat and subtlety reap the meager benefits
It's no surprise that combat and subtlety end up benefiting from Shadow Blades the most. Combat's heavy reliance on autoattacks and subtlety's potent finishers synergize well with Shadow Blades' design. However, these are two specs with quite the cooldown arsenal already.
In order to figure out how much Shadow Blades would affect our DPS, I turned to our favorite tank-and-spank encounter, Ultraxion. On a typical Ultraxion encounter, combat rogues are dealing about 30% of their damage through raw auto-attacks. That's quite high compared to assassination's 16%, though not much higher than subtlety's 24%. With armor reducing our autoattack damage by around 30%, the conversion of our auto-attacks to shadow damage should increase their damage by the same amount.
While a 30% bonus to your auto-attack damage sounds great, it's only for 12 seconds every 180 seconds, or about 7% of the time. When you do the math, it means that combat sees around a 0.7% DPS bonus from the shadow damage conversion, while subtlety is close with 0.6% and assassination brings up the rear with 0.4%. When you look at it that way, it's quite clear that Shadow Blades is a weak cooldown. By comparison, Vendetta is at least a 5% DPS bonus, making it over 10 times as powerful as Shadow Blades for assassination.
The combo point bonuses
The combo point bonuses don't fare much better. Assuming that each use of Shadow Blades results in one bonus finisher of the spec's choice via the extra combo points that are generated, we can figure out the relative DPS boost. Surprisingly, all of the specs end up valuing the combo points equally, with a 0.7% bonus to DPS for each. Eviscerates and Envenoms are similar in count and strength for subtlety and assassination, while combat's Eviscerates are fewer but more powerful.
I should note that combat and assassination have the opportunity to actually see a slightly higher DPS increase than listed above. Combat values combo points for more than just Eviscerates, due to the fact that more finishers results in more cooldowns via Restless Blades. Assassination rogues enjoy a nice bonus to their damage via the Envenom buff after their finisher, so that plays into the calculations as well.
Adding it up
Putting the auto-attack damage increase and the bonus combo points together, it's clear who comes out in the lead. Combat enjoys a 1.4% overall damage bonus, subtlety is close behind with 1.3%, while assassination lags with a 1.1% boost. When compared to other cooldowns like Vendetta, Adrenaline Rush, and Shadow Dance, it's clear that Shadow Blades is the weakest cooldown in our toolbox.
Our tier 14 set bonus halves the cooldown on Shadow Blades, which means it will end up doubling is efficacy. That means combat gets an extra 1.4% bonus, subtlety sees 1.3%, and assassination scores 1.1%. Those are below average for a four-piece set bonus, which usually give us a 1.5% damage boost. Assassination, of course, is way behind. My easy way of fixing this would be to make Shadow Blades simply give each class a 10% boost to their damage as shadow damage during its duration, which would affect each class equally and would completely balance the ability across each of the three specs, in addition to increasing its potency.
Sneak in every Wednesday for our Molten Front ganking guide, a deep-dive into the world of playing a subtlety rogue -- and of course, all the basics in our guide to the latest rogue gear.