Untargeted AoE can detect Ninjas in stealth mode.
AoE as basic skills can stop opponents from capturing a point (tomelith).

I believe AoE are nice but only if you're several to do it to overwhelm the enemy healer. If only one player spam its AoE, it sucks and the healer won't have that much trouble.

The problem of AoE imo is that it needs more coordination than using the 5sec countdown macro and burst someone with single target rotation in Feast if not in team (so usually with a voice chat system).
The result seems worth imo.

For exemple, DRG are great to burst in AoE with Stardiver(2400dmg), Geirskögul(2000dmg) and Naströnd(2400dmg) (+ basic combo) and Dragon sight to make all this +10% dmg and +10% to another ally who will also focus on AoE + Life surge (+20% dmg) on the 3rd strike of his AoE combo (1200base dmg).
Ninjas can stack 2 mudras and 100 Ninki to use Goka Mekkyaku twice (1600x2) in a row with Hellfrog Medium (1200x2), with Bunshin, it will actually make it deal +400 in AoE for 3 GCD (so +1200dmg) to easily get a 3rd Hellfrom Medium to use(+1200), and with Ill Wind inflicting 800 in AoE with Silence for 2sec, preventing people from healing with potions and healer if the healer also is Silenced, which is amazing if comboed with a Summoner using his AoE 2sec stun during his Phoenix phase. On top of that, Ninja can start with his Trick attack to set a 20% vulnerability on one target, and can finish the one healed the less by healer with Assassination, Feint and Smite.
Monks are not the best at it at all, but still can stacks their 4 GL for +20% dmg, Fist of Fire for +10% dmg and Brotherhood for +5%dmg to you and your team, and can use Enlightenment (1200 dmg) + Axe kick (1000dmg) to regain easily another Enlightenment with the help of Brotherhood on top of it. Then as Monk your job will be to finish quickly in monotarget the one that has not been healed efficiently by the healer (Tornado kick still available, Feint and Smite).
SAM has the best basic combo in AoE (kinda even with DRG doing 800/1000/1200) inflicting 1000 and 1000 instead of 600 and 800 for Ninja and Monk, and once with 100Kenki, he can spam basic combo + Hissatsu : Kyuten (800 dmg) (like 6times minimum if hitting at least 2 targets) total : 4800dmg) + Tenka Goken 1600 and Kaeshi : Goken (1600 again). Keeping 25 kenki to use once Hissatsu : Chiten (the counter stance) is really worth too, it ends to be kind of an AoE move if you're targeted by opponent team.


About tanks, War can spamm 10 times in a row their AoE GCD (1200dmg), also providing +5% dmg taken from all the ones hitted, and Nascent Flash makes him and one ally very safe as the lifesteal will be on AoE moves inflicting more overall dmg, so a lot of HP will be recovered from it.
DRK can lower healing effect for everyone
PLD is more safe and reduce what enemies inflicts.

About Magical Ranged DPS, having its LB is a good starter (maybe too obvious, but well) as it will chunk HP bar enough and will provide a +25% dmg dealt to all targets.
BLM can stack 3 times Foul to spam it with Flare before to get +10% dmg and Phantom dart for a +20% on a specific target on top of it to all your mates. Nightwing to put asleep instantly in AoE is great. And Thunder IV will be spammable with all those dmg done quickly.
SUM has an awesome Phoenix mode to spamm 1200 dmg instant AoE with 8y AoE size and a 2sec stun in AoE to combo with allies. They also have an AoE DoT as strong as the monotargeted one, and Wither that will break shields and heal regens.

I can't write for everyclass as I don't know them all. But well, there's a lot of things that could be done, but yea... that needs way more organization imo.