I think Quina is probably the most useless out of them all. Would the game have been significantly different without him? The only thing he did was (accidentally) lead you into Fossil Roo, if I recall correctly. Oh, and he fought Lich with Zidane. But I think that's it. His abilities and attacks were pretty useless too, and his strength fluctuated a bit too crazily for my liking. At least Amarant had Throw, which made him somewhat useful in battle.
Quina had a huge use: he made Zidane and all his pals feel better about themselves xD