After getting "Minwu" (why did they change his name?) after returning from the castle at the start of the game, I just keep him in my party and level up my character's stats.
Everyone is now at about 1'000 HP and 150 MP, all have Holy (only just bought), the best armour fresh from Mysidia and I've not even gone to Salamand or Bafsk yet to continue the story when Minwu leaves my party.
I'm not giving up all his high-level healing powers just yet!
Minwu's shedloads of White Magic keeps me going all the way to Mysidia and back, developing my characters and the rest of the game will be a breeze. Until I get tired of developing everyone and earning loads of Gil from selling Berserk scrolls won from the Magician monsters...