Flair's trance is better than Double White Magic in my opinion. Maybe I'm wrong, but ET said that before you get to level 99, you have to do something in order to receive their greatest stat (I can't seem to remember what is that something...).
Maxing Thievery is easier than maxing Frog Drop because you have to wait a lot of times, while battling is easier (At least, you can train in battle). In the beginning of the game, he proves to be most useful to you, and his trances already does 9999 damage (for me) in Disc 3.
What do you mean by Vivi's 'badness'? (*Mad*) He can already do 9999 damage if you got Eiko/Dagger to cast Reflect to him (Reflect x2) unless if you don't train him. He was one of the character to be able to do 9999 damage without going on a thousand battles for Dragon Crest, Thievery etc.