What're you talking about? Apart from Quina, Amarant is one of the most useful characters in the game, especially in tougher battles such as Yans and Ozma.
Amarant is one of the only two characters who can rebuff the party with Auto-Life after the initial ability has been used up the first time in a battle... the other being Quina. But unlike Quina, the ability costs less MP, has an added effect of Regen, and it can be used on the entire party when in Trance.
In addition to this, he also has a max damage skill (No Mercy) just like Zidane, Quina, and Freya, which is accumulated by the amount of damage he's done to enemies in battle.
And if that's not enough, he has an ability that changes elemental weaknesses, a resurrection ability (Revive), and a Doom ability (Countdown) that increases in probability of working as his level increases. Combined with Quina's blue magic spell Night, Countdown makes the battle with 3 Yans - which is the toughest and most rewarding normal monster battle in the game - a lot easier without the risk of having your whole party ejected (which surprisingly results in an automatic Game Over).