On the contrary, I (who happily use brew and brew cask) have re-purchased anything that is available in the app store, even if I'd already purchased it elsewhere, just for the app store conveniences.
It's really amazing to dump an old Mac for a new one and just click from your purchased inventory, no serials, no nonsense. Not to mention having that work on both your laptop and your desktop.
The old days of having to take a day off just to move to a new or additional machine are long gone, thanks to the app store.
1. Just one account for a wide range of software.
2. Automatic background updates for all purchases without having to deal with lots of sparkle updater dialogs.
3. Easy access to all your apps on any of your macs.
4. Easy reinstallation when you move to a new machine - no hunting around for license keys etc.
5. Confidence that the software is sandboxes and not malware.
Your claim that the MAS offers 'absolutely nothing' is false.