Eiko, Garnet and Vivi are shit, truth be told.I meant his appeal as a character, not mechanics wise Having said that, is he that good? I haven't played IX in ages, but aren't the mage characters any good?
A well built team of:
Zidane (thievery) - Freya (dragon crest) - Amarant (no mercy I think it was) - Quina (frog drop)
All do 9999 damage for lowest possible mp in a sustainable party. Quina (just about everything), Freya (heal hp & mp) and Amarant (recover mp, regen and auto-life ) can heal and do tons of other stuff.
The only one "hard" to build into the party is Amarant if you are determined to make him hit for 9999 damage as you need to use specific equipments when leveling in order to get his stats right.
Steiner is fairly easy to get to hit for tons of damage but doesn't really do anything outside of that and it costs a fair lot of mp
Eiko & Garnet are really just supportive characters and because of that they don't offer enough to join the party. Vivi is the same as Steiner, just less reliable) because all he does is damage, but not enough damage to make him challenge the others with their fixed damage and party utility.