I understand that saving the LB is sometimes needed for the fight, like the Ifrit nails, but I don't understand people who insist on saving it for the last minute on bosses where it doesn't matter.

So many times I've seen DPS not use LB on the first boss of AK until the last second, or sometimes not at all. Either you do the damage now, or do the damage in two minutes when he's about to die. The only difference is you could have been recharging the LB gauge for the entire boss fight.