Hey she's done stuff! Like that time she… um, did that thing! Which turned out to be her fault…
Or that time she used that ability that… made things worse in the end…
I mean she brought him back to life! No wait that was mostly that other girl.

Damn. She is useless.

It a case by case thing. In some they're all useless, in others they're awesome.

Megumin may only attack once, but that attack can kill almost any enemy. Making her perfect to finish off a boss or horde. Making her the ultimate glass cannon.

Aqua doesn't fight unless the enemy is a undead monster, but she's basically the white mage of the party. And who the hell sends the white mage in a fist fight? But she's the ultimate white mage. Need healing? Wounds healed in seconds. Died? Brought back to the world of the living. Need spare MP? She has tons of it. Undead horde? All gone with a few casts.
She's the ultimate white mage, with all the good ad bad things a white mage has.

And Darkness is the best meat shield someone could ask for. She might not be able to hit anything, but you can beat that she's make sure you don't get hit either.

And Kazuma is basically the brains & Red Mage of the team. He comes up with the victory plan and like all good red mages, he's decent at everything.