[Cerium]

The reliability of the printer versus the phone is not at discussion. The reason to go with paper tickets is that they have significantly better availability and partition tolerance than phone based tickets. Using a paper ticket requires that the ticket be available at an exact future moment in time (when you board). The hassle of potentially using multiple unreliable printers ahead of time to prepare the paper ticket is an engineering trade off in providing a system with excellent time of use properties.

In the event that your personal printer is not functioning, the airport has many publicly accessible printers for your ticket printing convenience.