Stripe can already store CC details. You can create users for each of your customers through the API and then build out your own "easy payments" system without having your users ever leave your site.
It's more than just payments within any given app though. The issue (/proposed issue) is that every time you join a new site or pay for a product on a new site you need to look up your details again. This isn't the case for sites using paypal, assuming the user has a paypal account which it seems most people do.