Let's say you're a company like Mint.com, or even Facebook for instance. You develop an app for the iPhone and Android. Chances are you're not gonna recoup the costs of producing the app (paying programmers, man hours, etc) from the app revenue.. I don't use many mobile apps, but I assume the Facebook app is free??
So why do companies make mobile apps? is it to protect their "moat"(their actual website where the ads are displayed) so to speak? To appease their users so they won't use a competitor?