But again, sac is perfectly viable on way more fights than just Primordius; and the rule of not doing irrelevant damage is still not contingent on that, it's just a general rule. Here's a list of what I do with sac, in my current progression:
Jin'rokh: nothing, GoSup for this fight
Horridon: LOTS of opportunities to RoF on adds (+boss) for extra embers, FnB for AoE on adds, Havocleave with SB/CB onto Horridon/relevant adds, leech embers with SB from adds
Council: lots of opportunities to Havocleave (either CBs or Immo/Inci/Conf for more embers), leech embers with SB (on Loa Spirits)
Tortos: FnB AoE on bats, Havocleave with SB to Tortos from bats/turtles, RoF on bats/turtles for lots and lots of embers
Megaera: RoF both heads for more embers, Havocleave Immo/Inci/Conf for more embers
Ji-Kun: pool embers for Primal Nutriment, Havocleave Hatchlings with SB/CB - not too big of an advantage, though, especially if you're not on nests
Durumu: GoSup normal, GoSac for heroic (RoF/Havocleave on ice walls)
Primordius: THE GoSac fight, infinite embers and pew pew pew
Animus: depends on strategy; single target damage on Animus is important, but if you have trouble with the "trash" phase you can Havocleave/RoF at several points; probably too variable for a blanket assessment
Iron Qon: GoSup normal; potentially GoSac for heroic due to Havocleaving when all mounts are up, though I have not tested this yet and might be wrong
Twin Consorts: GoSup; some Havocleave during Tears of the Sun, but not enough
Lei Shen: Havocleave with SB all through p2/3 from Ball Lightnings and random adds (these also p1), leech embers with SB from adds, FnB AoE on Ball Lightnings; can't talk about heroic, many seem to favor Demo
Ra-Den: no info
I find that GoSac is very viable on a lot of fights; that being said, there's also fights where GoSup is better (or going another spec altogether). Your mileage may also vary depending on what job you are assigned by your raid, what your raid composition/size is, and what fight you are progressing on currently. Always put the needs of the guild above your personal numbers!