As already mentioned, there's no reason to not register ALL of your credit cards in the program. If you register your AA Visa card and use that at a participating restaurant, you'll receive the miles for dining with a registered card (ANY registered card), and you'll receive whatever benefit -independent from the I-Dine program - that the AA Visa cards provide you.

So... is there an advantage? If you AA Visa card awards 1 mile/$ spent (as an example), for a $75 meal, you'd ean 75 miles by charging on AA Visa, PLUS miles by using that registered card at a participating restaurant (the number of miles earned through I-Dine is dependent on your I-Dine status ... there are 3 levels, IIRC)