Mind Blast and Shadow Orbs, revisited.

When, specifically, we should cast Mind Blast is a topic of contention among theory-crafting shadow priests. Many beginners make the mistake of only using it with 3 shadow orbs. This is wrong. However, among high-end theory-crafters, there are two common and opposed schools of thought, and a rarer third more detailed perspective.

The most common view is that Mind Blast should be used on cooldown, every cooldown. This is popular, because it's simple; no real thought required to execute it. My personal rule of thumb is cast Mind Blast whenever you have at least 1 shadow orb, and it's off cooldown.

My sims indicate that holding Mind Blast for 1 orb is a modest damage gain of 88 DPS (0.2% gain) relative to using Mind Blast on cooldown on a stand-and-burn fight. This effect becomes slightly more pronounced on fights with unpredictable interruptions from movement or other reasons (Fading Light, for example.) The effect is less pronounced, but still positive, when using the 4pc t13 with MSp/MB.

I recently forwarded this claim on a shadowpriest.com thread, and in my Advanced Shadow Priest DPS post. A commenter mentioned that you should be able to squeeze out slightly more DPS by using MB without shadow orbs when either Empowered Shadow will remain active for a while, or your DoTs will not need refreshing soon. However, in practice, this increases the difficulty of the rotation, and might offset such a small effect.

Nonetheless, I simmed it. The effect was miniscule; I required a 1M-trial run to get the margin of error to be smaller than the effect. My results indicate that this is a damage loss of 6 DPS (0.014% loss) relative to holding for 1 orb. True to intuition, this did increase Mind Blast damage appreciably, but small infrequent lapses in Empowered Shadow uptime offset the gain via lost Mind Flay and Shadow Word: Pain damage.

Conclusion

As I've stated before, both using Mind Blast on cooldown and holding Mind Blast for 1 orb are competitive, viable strategies. While you may claim that the increased difficulty of paying attention to orbs could offset the gain from holding for 1 orb, I should point out that my sim code never clips an in-progress Mind Flay to cast Mind Blast; in practice, the effect is likely to be slightly larger than my sims would indicate, and thus offset the loss from increased difficulty.

I still recommend holding for 1+ orbs.