For many apps where I wouldn't mind spending $10 to $20 in the App Store, I am probably not going to deal with all of the above to buy it directly from the developer.
However, I have no data to back up that claim, so I could very well be wrong.
I personally don't feel like you do because I use burner cards and emails to solve the hacking and spamming problems, and apps still have to be sandboxed unless you turn off that safeguard explicitly for the app.
Pretty much everyone takes PayPal which shields you from this
It's not necessarily a dealmaker or breaker; but if someone really cares about that and the market has many substitutes, it could be enough to sway them towards App Store purchase.
Example: Steam
I just don't think any implementation has really been that great yet. However as I think about Steam, it's rare I even consider a game I can't install via it - mostly due to it being an easy way to "back up" my games for later availability. That and they keep things up to date so it more or less Just Works.
Mobile app stores post-iPhone just took off. There was no 'slowly at first' phase, in the sense that they had an uncertain future. They went gang busters from the start. But we're past theorising about how the first 5 years of the Mac App Store might go. They've gone. It really doesn't look good and I don't see any signs it might improve any time soon. A few people appreciate the convenience, but as this case study shows it's not enough to make an economic difference to developers. Steam killed it, but the Mac App Store is completely stalled.
That could explain why someone chooses to buy it in the store if it is available both on and off the app store, but it's unlikely that there is many (any?) people out there who refuse to install an app that isn't in the app store.