Compare a BM+WM to a RM. The RM gets moderate fighting skills; totally worthless after, say, the Earth Cave, where the BM has enough magic to use a spell just about every turn for the rest of the game. The RM doesn't get all the level 7 or 8 magic; NUKE, FADE, LIF2, WALL, those are nice to have. RM has higher defense, but mages go in the back row, and they don't get hit that often.

If I want magic, I want as much magic as possible. If your mages are going to be sitting in the back row, there's no reason not to take a BM over an RM, because a BM gets more magic, and magic is the reason you take mages.

All depends on your style and your party though. I prefer specialized characters. A Fighter who only fights, a BM who only uses black magic, a WM who only heals. I don't like watering down my mages by trading some of their magic (their only purpose in the game) for the ability to wield the Fighter's left-over medium-strength weapons. RMs are good if you want a mage who is also a tank. RM/RM/WM/BM party is nice. F/RM/WM/BM is nice. All depends on what your party is. The WM/BM are far from useless though.