who gets the drop if you only need 2 melee dps?

[Alahra]

Kassatsu crits on each tick with Doton.

I'm aware.

Kassatsu isn't available constantly, though, nor is Invigorate, which NIN needs to reasonably sustain heavy use of Death Blossom, so fairly often, NIN contributes far less AoE damage than the other two.

MNK, on the other hand, delivers consistently high damage thanks to having both Invigorate and Purification to alternate for TP, plus Elixir Field and Howling Fist which have lower cooldowns than Kassatsu (and they can spike their AoE damage considerably with Perfect Balance for rapid acquisition of GL3 and Rockbreaker spam).

Though DRG doesn't have Invigorate all the time, the naturally high potency of their AoE attacks (and the slower attack speed) means that they don't need as much TP for AoE, making their AoE usually more consistent than NIN's as well.

Looking at raw potencies for all three classes doesn't really paint the entire picture, either, as both MNK and DRG have several self damage buffs that factor into their AoE damage (FoF, Twin, GL1-3 for MNK; HT, BotD), while NIN only has the one (poison) that only applies to Death Blossom. MNK and DRG also have more natively powerful DPS cooldowns (MNK gets 20% more crit from IR and DRG gets 15% more raw damage from BfB) than NIN does. Aggressively played for AoE damage, MNKs and DRGs can easily outstrip a NIN in AoE in most situations.