Quina worthless? HAH. You obviously never bothered to spend any time with her, since if you had then she would be one of the best. Take some time to learn her blue magic and catch frogs. THEN tell me what you think.
I'll give you a quick list of her best blue magic:
-LV5 Death (works on every enemy in the Desert Palace, allows for quick kills of Grand Dragons for easy EXP/AP gain)
-Mighty Guard (quick way to cast Protect and Shell against tough enemies)
-Matra Magic (makes Eating enemies much easier)
-Bad Breath (hurls a bunch of statuses at the enemy and ensures an easy fight, even works on a few bosses)
-1000 Needles (great source of unblockable damage early in the game)
-Night (used as a strategy against Yans although not the best, but still a useful strategy)
-Angel's Snack (simply awesome, THE best way to cure status ailments)
-Frog Drop (what makes Quina a damage dealer late game, 9999 unblockable damage)
-White Wind (decreases need for Eiko/Dagger when coupled with Auto-Regen)
-Magic Hammer (useful against magic reliant enemies and bosses)
Quina is far from worthless, and is one of the best characters. Zidane is the useless one. He has 3 (minus Steal late game) skills: Steal, Soul Blade, and Thievery. Thievery takes an enormous amount of steals to max (you pretty much have to have him steal every turn throughout the whole game). That is easily the worst fixed damage skill. Once you're done with Terra Zidane literally has no use anymore (except for Soul Blade) unless you took the time to max Thievery.