From our perspective as developers, the standard IAP model doesn’t work as well for prosumer level apps like ours. You don’t really generate recurring income from it, so you need to have a very large customer based — which Mac apps don’t really have — or you need to keep adding new features that can be bought. I think people get annoyed by having to purchase lots of individual features continually from a long list.
For smaller apps, traditional IAP works fine I think. But when you are trying to build up something bigger like Agenda, you need a way to collect a recurring fee in a way that doesn’t annoy the customers. For us, the Agenda model works well for this.