It goes without saying, but it's to do as much damage as possible while maintaining mechanics. In most cases, I'd say mechanics take priority over maintaining your rotation.
The thing about DPS as a role is that it can be very different from healing or tanking. Having played all 6 healer/tank jobs to 58/60, I can say that at their core, all jobs of those roles play similarly to the other jobs within their role.
But with DPS jobs, they're all unique in their abilities. Knowing how to play 1 DPS job does not guarantee you'll be good at the others.
In the case of a large pull, what you do as a DPS will differ based on your job. As a melee or ranged job, one that relies on skills that consume TP, you should try to AOE as much as you can, but don't spam it so much that you run out of TP. But if you're playing BLM or SMN, those jobs have specific in-built traits and abilities that ensure they can AOE all day without running out of MP, so they can go to town.
As a general rule, an AOE attack will always do more damage than a single-target attack, provided it hits 2 or more targets at once. So generally, even in a pull of 2-3 mobs, AOEs do more total damage, but be ready to switch to a single-target rotation when there's only 1 mob left.
