To be fair, of the last three years of owning an iPhone, I can only remember two or three instances where I was reading about an app and had an impulse to purchase it right then and there.
I do see/understand the advantage of having a deep-link on their homepage allowing me to purchase the app without having to launch iTunes (or the App store on my iPhone), but of the few instances (Camera+ was one and I forget the others) I didn't find the "friction" of having to launch the app store on my device much of an issue.
On that note, purchasing apps is more of a "casual" thing for me. When I'm interested in buying an app, it's usually done in "batches". And it's usually done when I've got nothing better to do (before bed, on the train, etc.) so settings where browsing the app store on the phone is the most comfortable.