Eiko's double magic is great. You can heal your party while causing damage to the enemies with Holy. Or if your team doesn't need healing, you can cast two Holy's to cause A TON of damage.

Still, I personally don't think Trances are that important compared to the rest of the character's abilities. I find Quina's trance to be most of the time useless but that doesn't stop him/her from being very useful in battle.