For a merchant who doesn't have any suppliers who accept Bitcoin, it's just an extra overhead without economic benefit: accounting, taxes, operational. He's better off buying Bitcoin on the side as savings.

So at this stage, you will only convince people who'd do it out of principle. Convince them with "Why Bitcoin?"

Once a merchant is introduced to a supplier who accepts Bitcoin, the landscape changes, but you still need to convince them to run a dual-currency operation, which is a big accounting headache.

However, at this point, the more "savvy" of them might realize the potential of hiding some of their cashflow from the government, which is a very enticing prospect. Convince them with "How Bitcoin?"