Thanks @gregvaughn and @easco for the shoutouts!. 
I personally prefer to think of Phoenix as a tool for building web interfaces rather than web applications. I try to avoid the term “Phoenix application” as well because I think it limits how we think about our approach.
In my book, that Greg alluded to, we build a fully functional application without Phoenix. That’s the first two thirds of the book.
Only when we want to make that application available on the web do we layer on a Phoenix interface. We keep the code for those two parts physically separate by means of a poncho project, as much to help the flow of the book as anything. That’s the last third of the book.
My quick response to the question:
You can definitely build a complete application without Phoenix. (I think doing so brings some strong benefits.) If you need for it to have a web interface, though, Phoenix is a fantastic choice.
