The main thing about quickenings is the fact that once you get past one you get more MP. For example if your character with 100 base MP gets one quickening he still has 100 MP, okay that sucks. But when he gets a second quickening it doubles to 200 MP. On the third quickening he gest 300 MP. Is that sweet or what?
I also like using quickenings not for boss fights but if you're overwhelmed by monsters, if you have a bgillion skeletons swarming on you (that happens in some spots) it helps a lot to throw a good ark blast (2 level 1, 2 level 2 and 2 level 3 quickenings at once) at them and wipe them out. The risk is if you screw up on the quickenings and dont pull off enough to get a big enough concurrence together to kill the enemies, you're going to be stuck with no MP and still a ton of skeletons facing you.
On a side note, if you want tons of MP but dont want to get quickenings for some reason, you could instead get a high powered esper for that character and it will boost your MP bars to the amount needed to summon the esper. So if you get zodiark (needs x3 MP) then you automatically get 3 bars...now I don't really know why you would want to do that, but the option is there anyway.
