shatterosul is the first staff WS that's worth a damn (that does damage anyway), and it also has a beneficial effect for mages.

The idea of a BLM meleeing during an 18 man/large party event is a bit absurd, akin to a DRK seriously attempting magical DPS. In different areas of the game, your job will act accordingly.
I wouldn't ever suggest such a thing, however, if the caster didn't have to lose TP, they could make use of TP accumulated from casting at oppertune times.