Pros: No need to enter shipping address, card info if on amazon account. It basically provides a simplified checkout process.
Cons: Amazon gets a commission of a sale they had nothing to do with. They're already in competition and hurting OWC this option gives them a slice even if you don't order from them as OWC/other sites have to pay to use the service.
Amazon knows you bought from OWC and the amount, who knows what they do with this info or what advertisers they share it with.