It would slow you down in the beginning of the game, but not so much for the last couple of bosses (starting around Giruvegan).

Quickenings are ok, you just have to know how to use them, and have your party set-up right. I always controlled Vaan or Ashe, and they never used magic, then I had one light magic user, and one heavy White Mage. So, when it got time for a Quickening, Vaan or Ashe always started with all their Mist Charges in tact, and the light magic user had most of theirs, so it didn't hurt me that much so long as I got a good chain going and killed the boss. If not, I just subbed out my healer and finished the fight that way.

You just have to use some strategy with Quickenings, you cannot abuse them like you can Limits in other games, but they are still very helpful, esp. with some of the early bosses who enraged when their health got critical. It let you put the finishing touch on bosses like (SPOILER)Ghis and Bergan before they could combo you to death.