1. I know a lot of Mac users that use the MAS as the only (!) source for their software (even Scrivener and other software has come to the MAS that before was only sold through the developer website). These are "normal" users that are not developers or hardcore nerds. And they never see any problems with sandboxing (they don't even know what this is). But - and that is the beauty of the MAS for normal users: Apple has their payment info on file, allows easy browsing the catalog, easy installation, easy payment and easy canceling of the order just in case. In addition: iTunes gift cards that are sold with a discount.
2. With the new VAT laws in the EU starting 2015, it will be a pain it the ass for small developers to sell software and services through their website on their own - at least for developers in the EU.
Of course, developers of business-related software for developers might see advantages selling on their own. But an exodus?