Find a ghastly example of all-caps text in a brief or formbook. Then read it closely to see how many typos you can find. If you're part of a writing group or class, bring a copy for each colleague.
In the literature on typography or on effective writing, find two authorities stating that all-caps text is hard to read. Type the supporting passage and provide a citation. If you're part of a writing group or class, bring a copy for each colleague.
Find a state or federal regulation requiring certain sections of certain documents to be in prominent type. Interview a lawyer who sometimes prepares these documents. Consider (1) how lawyers comply with the requirement--especially the extent to which they use all capitals, (2) whether capitals are actually required, and (3) whether you think there is a better way to comply with the requirement.