Reading this interview and quote from one of the cofounders of Stripe reminded me of what retailers often forget regarding apps:

Building an app is expensive
"Buying" app downloads is expensive
Customers are only willing to download a select few
Customers only use a select few

This is not to say retailers shouldn't build single purpose apps. Just remember that given the fickle use of apps, flocking to where the eyes already are makes most sense. Brands not using apps and marketplaces that show a multitude of brands won't ever be part of the conversation. When not part of the conversation, sales and awareness suffer. As Stripe Co-Founder Collison says:

Our idea is to let retailers enable buying in the apps consumers already have installed, rather than going on a quest to get consumers to install their single purpose app.