Some people like Final Attack but in general people avoid it because it decreases DPS with Slash Blast (250% instead of 6×130%=780% damage) - Basically lowers overall damage.
You are correct. FA is bad damage for multi-target. But Coma is even better for multi-target damage. FA is used for the utility of the ability, not the damage, as explained by Kami:
Fighters tend to spend their sp on final attack because they charge their combo faster, thats the only positive i've got in mind regarding final attack, that and as said before, bossing.
Fighters sometimes will dump 30 points into it right at level 70. It's useful for charging combo's faster, so you can use Coma more quickly. However, still better forgo FA early and simply choose to max it after Combo + Coma from levels 90-100; saving SP until level ~100ish. Coma training is best at DGP + gobies. Neither of these training locations are out yet, so its ideal for fighters to skip FA for now, and simply come back to it later in 3rd job when these areas are released. No other sader skills are really that important, except maybe panic; but even then you will never use it in training. It's simply not worth wasting your combo orbs over unless you're fighting a boss.
However In those early versions of the game its mainly about grinding and lvling up, and final attack actually reduces exp per hour ratio, hence I dont see it benefiting White knights as much as Fighters (faster combo charge = greater damage).
Yup. FA on White Knights is pretty bad, because there is no utility for it. One of the reasons in which FA increases bossing damage is faster Panic charges. A significant chunk of your damage will come from Panic. White Knights don't care about that, and therefore should really just skip it completely. The slight buff in DPS you'll get from FA while bossing is not worth it in grindstory for WKs