Whoops. 
Sorry!
But, more on topic, not realizing this was an old thread, I spent some time contemplating FA while I was training.
Now, I'm level 68, so not quite 3rd job, so your results may vary, but I found that very rarely was I training anywhere that the damage difference found in using SB on a mob of 5-6 really made a difference. Moreover, due to my short range compared to a Spearman, on the majority of the monsters I'm training on, I'm not getting 5-6 in a mob, either. Maybe at MP3, the initial mob on a platform might be that big, but the difference would be negligible. To me, at least, they'd die in the same number of hits. But once that big mob is gone, you're left with groups of 1-3. And at places like V/hoodoos, or even Windraiders to some extent, Power Strike is more effective. So at least, for me, Final Attack was worth maxing. And as was pointed out by others, FA won't activate on 4th job attacks, so it's really moot by then.
Sorry!But, more on topic, not realizing this was an old thread, I spent some time contemplating FA while I was training.
Now, I'm level 68, so not quite 3rd job, so your results may vary, but I found that very rarely was I training anywhere that the damage difference found in using SB on a mob of 5-6 really made a difference. Moreover, due to my short range compared to a Spearman, on the majority of the monsters I'm training on, I'm not getting 5-6 in a mob, either. Maybe at MP3, the initial mob on a platform might be that big, but the difference would be negligible. To me, at least, they'd die in the same number of hits. But once that big mob is gone, you're left with groups of 1-3. And at places like V/hoodoos, or even Windraiders to some extent, Power Strike is more effective. So at least, for me, Final Attack was worth maxing. And as was pointed out by others, FA won't activate on 4th job attacks, so it's really moot by then.