Pure theory from me (some are a little bit obvious too)
Also, i can only speak for SMN in HL content

Balance :
In general, the more damages output there is, the more this 15% buff is effective.
I would say it's good on Melee DPS on bosses, and good on BLM on adds phases.
It would always be more effective if you apply this during burst phase of the DPS.
SMN case : SMN deals more damages with dots. If you use this buff on SMN while he don't have to reset the dots, it might not be worthy.

Bole :
Tank burst or simply during strong aoe if someone of your party has death debuff (ex: gigaflare on t13)

Arrow :
physical DPS

Spear :
BLM during fire cast phase, SMN during transe-bahamut phase (for more ruin 3 spam)

Ewer :
BLM would be able to cast more fire with this (unless this dumbass opens with flare ._.
SMN doesn't really need this but it can help him to raise or summon pets after being raised himself

Spire :
Melee during AOE phase

no feed back yet