My biggest suggestion is simply this, if you want to know how useful a job class is, make Ramza the class. He starts off with all around above average stats and he's actually incredibly good as either a mage or warrior. His Squire class is actually ungodly powerful in the later parts of the game.

Ramza can pretty much show you how useful any class can be and while some classes are much more exploitable than others, I don't think there is a bad job class in this game if you build the proper set up. There are a ton of story character to recruit in this game but my advice is to try to keep a few of the generics as well.