Dont bash the BM, I could never lvl as fast as I do now without my BM with fir2 and ice2 in the pravoka fingertip. And yes, it is useful in the rest of the game too.
I dont see any real reason to take a BB over a fighter, the fighter is useful in the beginning and stays useful. The BB is just about worthless in the beginning (but.. uhm.. cheap to equip) and doesn't really exceed the fighter in anything else than damage (however, to see a real difference you have to be overlvled).
The WM can be good, but with a bit of tough luck it will use most of it turns to heal itself. In most cases I'd prefer a RM.
Never used a thief (oh, it can run? how convenient, well my fighter gets away in most cases anyway)

Right now I'm playing with F, RM, BB, BM. And at lvl14 is doing the same damage as the fighter, however this will probably change as soon as I get my hands on a coral sword or something.
But, sure, I could use a BB sometimes, you've only got 6 characters to choose from, and it's funnier to greatly vary the party than to replay with essentially the same party.