Do auto-attacks serve any meaningful purpose?

[Iscah]

But we have an example of jobs that don't do them - casters, because they are, well, casting. SMN, not so much casting but still technically a caster so the autos don't happen.

Even for a full-time casting mage like a low-level healer, you get auto-attacks hitting after every second GCD or so, and at low levels it is actually a reasonable amount of damage per turn. Not a lot, but enough that you're probably defeating things slightly quicker than you would otherwise, especially if solo.