The in-app purchases are perfect for this. You get to demo the brushes before you use them to figure out if you even need or like them. They're asking what amounts to what -- $10? for a beautiful app that looks and works worlds better than most in the same arena. I much prefer paying nothing up front and being able to try all of the features and figure out which ones I want to buy. I ended up getting them all simply to support the concept. You cite Notability but I don't want an app that does 20 things, with an ugly/more complex interface to boot. I have other apps that excel at those other things if I need them. I just want a drawing app, and I'm willing to pay a premium for one that offers the best experience for what I'm trying to accomplish with it.

I'm tired of paying for apps up-front only to find that they weren't what I was looking for or what was advertised. Good reviews don't necessarily mean that it is the right tool for the task, and forcing the user to then have to request a refund instead of just having a trial period or a free version they can simply delete is not user-friendly and results in all of the "DO NOT PAY FOR THIS APP!!!!1" and "I WANT A REFUND!" reviews from people who know none-the-wiser (although Apple doesn't make it very clear/easy to do so to begin with) and cause more people to turn away before giving it a shot. It also allows for the developer to be able to persuade initially unsatisfied users with later [demo] releases.