Blade Flurry is broken
There are a few possible balance options for Blade Flurry, but none of them are perfect. Consider the first scenario, where combat rogues do good DPS without Blade Flurry active. Once they turn it on, they'll be doing good damage to two targets, putting them way too far ahead of everyone else. In the second case, their single-target DPS is awful, but they're viable with BF activated. That's where combat rogues are today, but we're so reliant on having multiple targets to attack.
Blizzard can't design every encounter to have a convenient cleave target for us, and it hasn't. Except against Halfus and Magmaw, combat is inferior to assassination in every way, and that's not balance.
Applying penalties to Blade Flurry
The third scenario is where combat rogues have good single-target DPS, but where Blade Flurry cuts down our damage by a more significant amount than today's 15% energy penalty. If the penalty is applied to our Blade Flurry damage, such as cutting it to a percentage of our primary target damage, then the net effect is that BF is less important than it is today.
The issue with this design is that we typically want the majority of our damage on our new target (such as an important add) rather than the old target. In order to put more damage on the add, we're switching targets, which means losing Bandit's Guile and our Deadly Poison stack, which murders our DPS. One of Blade Flurry's biggest benefits is that we don't have to switch targets in order to hit the second target, preserving the advantage of the buffs/debuffs on the primary target.
The fourth scenario is where a larger penalty is applied to our damage against the primary target, with the end effect of splitting our DPS evenly between our two targets. I think this is probably the least awful choice, as it allows us to stick on our primary target while still dealing respectable damage against the secondary target.
The problem with the design is that PVE fights are so varied in their design. Some bosses and adds are designed to be killed slowly or simultaneously, making an even-split Blade Flurry ideal. Other mobs, typically adds, need to be killed swiftly, which again means a target swap. Doing 50% of our DPS against an elemental on heroic Cho'gall simply isn't enough, so we're forced to take the target swap and deal less damage against both due to the lack of debuffs and Bandit's Guile on the add.
There's a fifth scenario possible, where our primary target damage is cut to say 20% while the remaining 80% is transferred to our secondary target. We get to stay focused on our primary target, gaining the extra damage from the already-present buffs and debuffs, but we still get to do serious damage against the new add. In all honesty, 80% of our primary target damage is probably 100% of our damage if we were to switch targets and be forced to reset Bandit's Guile and wait for debuff applications that may never come. Combat rogues now have the ability to "swap targets" with little downtime. The design isn't perfect, especially since doing most of your damage to something other than your primary target isn't exactly intuitive.
Blade Flurry needs a nerf
In all of the scenarios I outlined above, Blade Flurry loses some of its potency from today. The fact is that it needs a nerf; otherwise, combat rogues are going to remain one-trick ponies. Combat rogues deal the majority of the damage as physical damage, and so today, Blade Flurry is boosting our DPS by around 60-80% while active. A DPS swing that drastic can't be allowed to stay in the game. If combat rogues were only 10% behind assassination rogues on single-target encounters and Blade Flurry only increased their damage by 20%, I think that ratio would be relatively balanced.
As it stands today, there's only one viable spec for 10 out of the 13 bosses in tier 11, and that's unacceptable. I think we've accepted as a class that subtlety is going to suck in PVE, but seeing combat down there with frost mages and beast mastery hunters isn't helping anyone. The design goal of "bring the player, not the class" is in direct contradiction with combat's current bipolar potential.
We can either wait for combat rogues to break some heroic encounter so badly that the developers are forced to nerf them, or we can hope that the Blade Flurry redesign is done proactively to ensure that the spell isn't simply nerfed into the ground.
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