Your airline won't issue a mobile boarding pass if you are leaving from an airport that doesn't have the scanners. Some airports have very user-friendly scanners at the TSA check point, but others (notably Seattle) don't, and the checkers have to go find one, borrow one, etc.
My own personal take on it is that the mobile boarding pass is close but not 100% there yet. It's good to take a paper backup just in case.