No, I don't think Trance is useless. It makes Zidane a really good fighter with all of his Dyne skills doing 9999 damage by the end of the game. Steiner will almost certainly do 9999 damage with all his physical attacks later in the game when he's in Trance. Dagger summoning the Eidolon when in Trance helps you deal some extra damage, not to mention I've noticed Auto-Regen lets you gain a lot of HP back while the Eidolon in the background is using its attack, especially if the Eidolon is Bahamut or Ark.

Vivi's Doomsday x2 is very useful if your characters can absorb it because that will certainly heal all your characters fully and deal almost 20,000 damage to all enemies. Amarant can target all allies with his Flair skills when he's in Trance, which is useful since you can cast Auto-Life and Regen on everyone, revive everyone or heal everyone's HP and MP. In addition, I think Trance rids you of all status ailments. That's why it's a good idea, for example, to have Amarant near Trance before you fight (SPOILER)Ozma, and when he hits your characters with Curse, Amarant will reach Trance and be able to heal everyone with his Flair skills.

Although Trance has its uses in battles, I'll still agree with what was said above that Trance is more important to the storyline than gameplay. I wouldn't say Trance is totally useless at the moment I see (SPOILER)Kuja using Ultima to destroy everything in Terra when he finally reaches Trance.