maybe the phone thing is in tandem with a photo ID. I can see that being better than just the ID alone, but not without the ID at all.
Certainly when you are going through the security line and are "greeted" by the TSA staff, you are required to have your boarding pass and ID present. Whether that boarding pass is virtual as in a PDF displayed on your phone or a piece of paper you received from the ticket counter.
I just wonder how effective showing that phone with that open PDF is to TSA. How much easier is it to photoshop a realistic boarding pass when the PDF is simply Time New Roman printed information and a scrambled maze of dotmatrix goodness for a scan code that the TSA doesn't even have to verify works? That at least gets you into the terminal. And where on your virtual boarding pass does the TSA physically sign with a marker that he or she has verified it?