I don't end up using 95% of the characters abilities by endgame, despite wasting hours learning them. Heh... Anyway, most abilities are too specialized to be practical, but there are a few that are idiotic wastes of time no matter how the player slices it. The worst would probably Mag Elem Null which costs a great deal of magic stones just to make elemental magic non-elemental. Steal-Gil is another piece of crap, since the only time extra gil is needed heavily is in the early game, and the formula for the amount grows the higher Zidane's level is. All in all, near-worthless. Stuff like Six Dragons would fall into this category, but at least its outcome can have a positive effect as a last-ditch effort.

None of the characters' abilities are really meant for long-term use, though, except maybe Eiko's white magic and summons (and Dagger's to a lesser extent).