Opportunity costs are lost, not paid
If you go to the innkeeper in Orgrimmar and buy a Massive Turkey Leg, you just paid a cost for that food. Two gold pieces have left your pouch. When you expend 39 energy to use a Sinister Strike, your cost was 39 energy. Eviscerate has a combined cost of both energy and combo points, so you are missing both energy and combo points afterwards. Simple costs like these are easy to comprehend, and we deal with them every day.
An opportunity cost is what you lose by choosing one thing over another. Let's say you're sitting on 5 combo points, and you need to figure out what to do next. If you cast an Eviscerate, you're going to see some damage immediately. But if you burn your combo points Eviscerate, you can't use Slice and Dice for a while. When you're faced with a decision, the opportunity cost is what you would have gotten if you picked differently. Every choice you make involves sacrificing the other options, and the opportunity cost represents that sacrifice.
In that example, the opportunity cost of casting an Eviscerate is missing out on casting Slice and Dice. If your Slice and Dice buff still had 30 seconds left, then the value of using Slice and Dice again is low, so the opportunity cost is low. If your Slice and Dice was about to fall off, then the opportunity cost is high, since the Slice and Dice would be incredibly valuable.
To find out the true value of your Eviscerate, you have to weigh it against the opportunity costs of other options. If the Eviscerate would boost your overall damage done by 100k and using Slice and Dice would increase it by 200k, then wasted value of Slice and Dice outweighs the gained value of your Eviscerate, and you've made a bad decision. If Eviscerate is worth 100k and Slice and Dice is only worth 20k since it still has a long duration left, then Eviscerate is the better decision. You'll deal some damage with either choice, but by weighing your opportunity costs, you can find the best solution.
Rotation guides hide most of this math
When you read a guide on Elitist Jerks that tells you to prioritize Slice and Dice over Rupture for combat rogues, what they're really saying is that the opportunity cost of using Slice and Dice is less Rupture uptime, but that the opportunity cost doesn't outweigh the benefits of Slice and Dice. When you read that the best time to use Revealing Strike is when you're at 4 combo points, it's because all of the alternative options have higher opportunity costs.
Losing energy via capping isn't always bad
Synchronization of cooldowns has been one of the paragons of combat's DPS model for years, and that holds true to this day. If a shaman in your raid just popped Bloodlust, you should pop all of your cooldowns at the same time to achieve the best effect. Stacking Bloodlust with Adrenaline Rush and any trinkets or potions you have will maximize your damage done. The only issue is that by activating Adrenaline Rush at the same time as Bloodlust, you're likely to run into an energy capping situation.
We have a choice to analyze here. If you pop Adrenaline Rush and your other cooldowns during Bloodlust, all of the extra attacks will generate a massive amount of damage. However, the opportunity cost of activating both AR and BL together is all of the energy that we can't spend while we're energy capped. We could activate AR separately before or after BL to spend all of that energy, but we'd miss out on the cooldown synergy.
The key to solving this equation is to look at the big picture. Activating AR and BL together grants us some amount of extra damage, while our capped energy bar represents an opportunity cost. We simply need to do the math to figure out if the AR/BL combo's bonus damage is worth some unspent energy. After crunching the numbers, we know that the synergy between AR and BR outweighs the opportunity cost of using AR separately, and so we can make the right decision when the time comes.
In another example, let's say we've just activated Adrenaline Rush while Bloodlust is active. We're going to be energy capped for a while. If we then choose to use Tricks of the Trade, we will activate our tier set bonus that reduces our abilities' energy costs by 20%. Obviously the set bonus is wasted, since we're not being limited by energy at that moment. Our target will see a 10% damage bonus, and that's it.
The opportunity cost of using Tricks during AR/BL is that we won't be able to use it right after Adrenaline Rush fades, where we could benefit from the set bonus. However, if we decide to save Tricks of the Trade for after Adrenaline Rush, the opportunity cost is that we might cast one less TotT over the fight than if we used it on cooldown every time. Every action we take needs to be balanced against all other possible options in order to find the optimal choice.
So when is it right to use Tricks of the Trade? Is it worth it to stagger our cooldowns to avoid energy capping at all costs? The answers to these questions are tough, and the right choice depends heavily on the situation. Your goal is to realize that energy capping during a cooldown phase isn't necessarily a bad thing, unless there was a better way for you to use that energy. Using Adrenaline Rush during Bloodlust isn't wrong if there was no better time to activate it. Figure out your options, weigh the costs, and then pick the right choice.
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