BMs get lots of casts of level 1 and 2 spells (FIRE/LIT/ICE), which should replace physical attacks for them in the beginning. After Elfland or so I never run out of magic for my BM. Then the CatClaw makes BM an OK fighter later in the game. That leaves a very small part at the beginning of the game where they tend to suck, but everyone sucks early in the game.

Strictly speaking if you devote an RM entirely to Black Magic they're probably better than BMs in terms of stats. But Black Mages have a coolness factor that makes them worth using even if they are frail. The game isn't about stats, it's about fun. This is why Black Belts are still able to suck even though their stats approach or exceed a Fighter's late in the game.