For MNKs and DRGs it sometimes makes sense, since they have a combo system. For example DRGs basic 1-2-3 combo ends with a 300 potency attack which would be kind of wasted if you unleash it on a mob with that low health. Same with combos that end in a dot like MNKs demolish. It probably makes less sense for ranged dps though (with the exception of SMN).

Sounds like maybe their judgement of a mobs health was a bit off.