Why waste a good general even if he is in a poor unit. A yari or even teppo ashigaru taisho is not the end of the world so long as he still provides the honour bonus. He's better than nothing. You just have to keep him out of trouble. At least it does mean that he is not going to be involved in the thick of its, and probable unlikely to die in general melee and send your army running. If he does get killed then its probably your fault for letting the enemy get to him. I think it worse to have the taisho in a monk, nodachi or yari cavalry unit where the temptation to charge into battle is more likely to get him killed.

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"Put 'em in blue coats, put 'em in red coats, the bastards will run all the same!"