Admitted...Garnet is a relatively week character, and her trance is virtually useless compared to Eiko's. Eiko does have some very good magic and summons compared to garnet too.
But I still keep Garnet in my lineup instead of Eiko every time I play. She may not be strong or efficient with trance, but she's very useful with typical spells like cure and protect.
Useless trance? COMPARE TO EIKO'S?
Garnet's Trance causes extra damage for NO MP.
With Eiko's trance you can use 1 more spell in the same turn with no MP reduction. If you mean you can cast 2 Curagas, well, you don't really need that much healing. Especially by the end game (Auto-Regen takes care of most of the healing anyway).