My first time through, I did not abuse the "flaw" you're mentioning. However, since then, I have learned it's much easier to master a job class when you do a few things. Just enter a battle with everyone as the classes you want to have them master, but with "Basic Skill/Guts" and make sure you know Accumulate. Oh, and you should have at least one Knight*. Kill everyone except one guy, then power/speed break the heck out of him until he hardly gets a turn, and then only does like 1 damage every turn. Just have everyone accumulate after that for about 45 minutes straight, and everyone will have a few thousand JP to spend.

To the newbies of Final Fantasy Tactics: DO NOT DO WHAT I JUST MENTIONED YOUR FIRST TIME THROUGH. A huge part of what makes this game fun is how challenging it gets, and trying to strategize and make your team work for you, even against tough bosses. If you use the technique I presented, you'll make the game way too easy and you will miss out on a great deal of what makes this game so freaking amazing.

*Knight can be replaced by any job with the ability to incapacitate an enemy, such as Leg Aim/Arm Aim, Beowulf's "Chicken", and many, many others