I think maybe you misunderstood the question.  You made a great case for why it is important to use mac address learning. The question asked however is why on earth would anyone disable mac address learning on purpose?

JSAH,Can you have another look at the question asked. For what reason or reasons would you disable mac address learning?You have made a good case as to why it is important to use mac address learning.The question is why on earth would you disable it...