Most stamina builds (and even some magicka builds) incorporate Minor Force into their builds already through the use of Trap Beast (Rearming Trap). In the event that this is true for your group(s), you don't really need to provide it. In that case, protecting the healer is the logical option.
However, the fact that in needs to be slotted on both bars poses a very important question: Is it worth it, or can the healer handle themselves without it? Can I use those two bar spots more wisely? In the majority of cases, at least given my own experience, the healer doesn't need to be protected, as they can usually take care of themselves through their own skills (Major Ward/Resolve through class skills, overhealing, shields, etc).
Opinions will differ, surely. My own opinion is that you can afford to use something else. When Guard was first re-vamped a few months ago, it quickly became the go-to for dps boosting, and then we all realized that Trap Beast is far more efficient and effective. At this point, my raid group very rarely uses it, and when we do, it's only for niche-type scenarios, and almost never is it on the dps or healer. Its most common use (as far as I know) is protecting a main tank during hard-hitting boss encounters, like the Warrior on hardmode. It's not very common anymore in a pve setting, but it still has it place. There are uses for it in pvp as well, particularly for small-group or duo settings in which there is a "duelist" and a pocket healer.
At this point, I'd say take Mystic for the healing boost, and if being use for pvp, I'd urge this consideration further. But I think you can afford to use other skills instead.
Thank you for that illuminating response. I'm just at a point where my rotation has been set for so long I'm growing bored and trying to find new ways to increase my utility. I might use it in pug groups to keep nubs alive.
As a Tank I do not use it and rather others not use it unless they're struggling to survive (which may or may not be my fault). This ability is definitely good for PvP but in the PvE scene I cannot see a viable use other than protecting your healer but that'll be an added burden + two wasted slots (if you need to change bars).
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Idk, something about the fact that 30% gets reduced by both defense calculations gets me excited. If only I could make someone else run it and save me the slots. Reminds me of the mitigation pets give witch doctors in d3. I'm interested in seeing how it stacks with mist.