Many thanks for an informative discussion on the merits of building or not building an app.
From a security perspective, there are a couple other points to consider: (1) building security into the development phase which protects the user confidential data, and (2) the risk that a popular app could wind up with rouge versions in the app marketplace which would damage brand credibility after much was invested in developing and maintaining the app in the first place.
NIST released a standard for vetting mobile apps and the entire standard can be read on the NIST.gov website at: http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-163.pdf