Thanks for your reply.
1. I pay no fees on my credit card. My full balance is paid automatically and 1-5% is refunded (depending on where I spent the money). To put that in perspective, if I spend $50,000 I get back $500-2500, which is not an insignificant amount. You are absolutely correct that rewards are subsidized by merchant and banking fees. Please correct me if I'm wrong, but don't merchants have to pay fees when accepting Visa debit cards as well?
2. Cash-back rewards only make personal accounting more complicated if you calculate them on an ongoing basis and pay additional fees for the credit card. If you do neither of these you'll stay within your budget and be rewarded with a substantial bonus every month that you can then spend or invest.
With all that said, I agree that keeping it all in one place would be much easier and I would love to try Simple myself. I just can't imagine replacing my day-to-day purchases with a debit card, no matter how awesome the system that surrounds that card may be.