Can't have multiple discounts in the cart? Splution: create custom discount codes on the fly for absolutely everyone in real-time.
The benefit of App Stores is getting a access to a mass number of qualified customers anxious to add compelling features to the platform they bought, be it a web store front or iPhone. The negative is the platform has to always keep moving ahead, and sometimes your app is right in its path.
We used to call it picking up nickles in front of the SteamRoller back in my Photoshop Plug-ins days. Was very frustrating, but still very lucrative. I have often talked about giving a demo of my next brilliant plugin to the Photoshop team lead and product manager just before the release of Photoshop 3. Three weeks later Photoshop3 was released with their version of my plugin in it.
Stealing the good ideas and adding them to the core platform would generate more money short term but would destroy the app store