I usually like to utilize every job/character when I play an FF, and Blue Mages are always alot of fun.

Like it's been said, it's all up to the player how useful you want Quina to be.

And about her physical attacks, I wouldn't say they suck as much as it's just that they're really random, like axe-wielders in other installments. Alot of the time it's sub-adequate, but I've seen her hit for more than my other characters at the time. I can understand not liking it for being unreliable, but sometimes you gotta take a chance in FF.

At the same time, if you prefer other jobs, that's fine, too. It's all about how you play the game.