Quick immediately gives you two turns without taking up any battle time. You can't use Quick to gain even more turns during these turns, though. At 99 MP, it's hard to make frequent use of Quick. With an Economizer, you could cast any two Spells in succession for 3 MP. If you had a Gem Box as well, that's four Spells for 6 MP. In combination with Ultima, this can get out of hand very quickly.