In my opinion Musashi's argueing for dual wield had mainly philosophical reasons. If you read his work then it becomes obvious that one of his main ideas was not to develop a preference. On the battlefield a commander shouldn't generally prefer one troop type but equally employ infantry, ranged and cavalry, only depending on the circumstances. Becoming to attached to a single troop type or tactic makes one predictable and vulnerable - it becomes a weakness.
A similar idea seems to be behind using two weapons with both arms. That way the weaker arm is constantly trained to wield a weapon on his own and the warrior can still fight well even if his strong arm is incapacitated. Again, the main goal is to overcome the weakness of having a preference, a weak and a strong arm.
It seems to me that this is a very elite and idealistic thing, probably unrealistic for the common warrior. He is better off taking every opponent seriously and expecting him to be on an equal level. Dual wield would probalby weaken the warriors blows and parries which is especially bad against an armored foe. I just don't see a place for it on the medieval European battlefield.
