The argument generally revolves around shooting a "ONCE IN A LIFETIME" "MISSION CRITICAL" event or person or launch. How can one be a professional videographer using cameras with ONLY one SD card slot? ( I wonder how the old guys got two Beta SP tapes crammed into a single Betacam.....).
I can't help photographers who want to shoot raw still images with fault tolerant redundancy but I can help all the hapless Sony and Panasonic (not counting the new GH5...) owners who feel helpless and vulnerable shooting only to their one bare and rickety internal SD card!
If you shoot with an Atomos Ninja Flame or the Shogun model of external video recorder you can default to 8-bit capture and send video to both the camera card and the recorder's SSD. You'll have the back-up video you've been pining for over the years along with the bonus of having a great monitor.
Problem solved. At least for video. Might even be a hack to record still images on your external recorder. I haven't looked into that yet. Growth market for Atomos?