That way you can still get FPV even if the app goes out and you have full control through the RC. Treat the app as optional.
I don't want to get too far off topic, but that got me to thinking about HDMI and a separate monitor just so I can avoid any potential for app crashes, which my stupid iPad does occasionally. I don't have problems with video lag, but I also don't fly all that far away (3000 ft is my farthest so far, and its usually less than 2000). Question, does the HDMI video out contain any telemetry? (Guessing not since that probably takes app input)