Wil Shipley gives a well researched article on the necessity of paid upgrades
in the Mac App Store.
For developers, upgrades fund the new software we’re writing, not the current
stuff. Think of upgrade revenue as the Kickstarter project for our next
product, whether it’s a major version of an existing product or something
totally new. We live for those upgrade humps. They keep us warm in the cold
Of course, this should also be adopted on the iOS App Store.