I'm noticing a rather large flaw in the arguments here.

At 6 Enemys (which you almost never hit with Doomspike, but if u sure u can use doomspike!) you already delt 11.160 potency with RoT.
In a pull of 5-6, every single class benefits from using AoE. Death Blossom may be weak, but it's still 100 potency. 500-600 is only rivaled by Mutilate, which takes a full 30 seconds to match what you can do in ~2.
The flaw here is that we're looking at the damage per total. And it is true that the TOTAL damage done increases with each use of a skill on a greater number of enemies. Where the flaw lies, though, is that the amount of damage done per enemy does NOT increase with the number of enemies hit with a skill. So, when we're talking about how long a fight lasts when using AoE abilities, it's not on the total damage done, but the damage done per enemy.

Think of it like this. NIN has an AoE skill that does 100 potency. Let's assume, for the sake of discussion, that translates to 100 damage per use, or 40 damage per second (assuming a 2.5 second recast time). So, if we took an enemy that has 1000 HP, it would take 25 seconds for the NIN using the 100 damage skill to take the enemy down. Now, if there were 12 enemies all with 1000 HP, it would still take 25 seconds to knock all of those enemies down at the same time.

On the other hand, a single target combo has a total of 670 potency, which I will translate to roughly 90 DPS (670/7.5 assuming 1 potency = 1 damage). So, that same 1000 HP enemy will take about 11 seconds to take down. And in a group of enemies, that translates to 11 seconds per enemy, so if there's a group larger than 3, then single target will take longer.

The other factor to consider is that using AoE abilities is a major TP drain, and that TP starvation is a major issue. Going back to the NIN example, the AoE move costs 120 TP per use. And assuming a 2.5 recast, that translates to 48 TP used per second. TP regenerates at 60 TP per 3 seconds, or 20 TP per second. So, this translates into a net loss of 28 TP per second. This means that, assuming the fight starts with a full TP bar, the NIN will run out of TP in roughly 36 seconds. If the fight lasts shorter than 36 seconds, the NIN will be fine. After 36 seconds, the NIN will be TP starved, and the DPS will drop dramatically.

On the other hand, single use will take a very long time to run out of TP. The total amount of TP used in the combo is 170 TP, which translates to roughly 23 TP per second. Compared to the 20 TP gain per second, that equates out to a net loss of roughly 3 TP per second. The NIN could go about 333 seconds before going into TP starvation.

I'm not saying that using AoE is always worse than using single target, but it depends on the situation. It depends on who the other DPS is, it depends on how much actual damage is done per enemy, it depends on the size of the group, and most importantly, it depends on how long each fight lasts.

I would be hesitant to make blanket statements.