There are only three reasons people might buy something from the mac store vs. from the developer directly:
1. To keep it getting updates through the app store vs. having to download from the developer when a new update comes out. 2. They found it in the mac store, and thus purchased it in the mac store. 3. They trust apple to reduce the risk of malware when using the mac store vs. a developers web page.
The mac store is getting really hard to navigate, and it doesn't give any useful search functionality, so #2 is disappearing as a reason people are buying from the apple store. #1 is nice, but with auto updates, it is disappearing as well. #3 isn't generally an issue for developers or "power users", so it is gone as well.
Honestly, I can't think of a reasonable reason to put something in the mac store that is oriented towards developers or power users. The hit in revenue just isn't worth the benefits. Though I would love to hear from someone who came to a different conclusion.