Ok, but sarcasm aside - from shooting with ML's dual_iso I have a vague idea how much dynamic range daylight high-contrast shots have. I'd say with 15-16 stops you should be really covered for most natural scenes, 14 is fine for most purposes unless the sun is in the frame. So what's 21ev for?
Flexibility. I can always add contrast and crush the ends of my levels to turn it back into 15-16 stops, but its much harder to turn 15 stops into 21. Could be especially useful if, for shutter speed reasons, you need to under or over-expose signficantly. I dont mind clipping highlights if I wasnt gonna use the top 3 stops anyway, if that's what I need to shoot at the right shutter angle.
It's mostly a question of "why not?". Do I really need
to be able to shoot cleanly at ISO6400? No, but, I might be able to save a lot of money on my production using an A7s cause I need less light.
edit: Other thing that occurred to me...it could, combo'd with the Movi system, etc...completely change moving from indoor to outdoor for filming. If leaving a bright outdoor scene and moving indoors I change 7EV, with a 21 stop sensor, I might not even have to pull exposure as I change between those scenes. I can just adjust my color grade in post.
Of course, as Red Dragon showed, "21 stops" is never actually 21 stops...it'll be 17 or 18, which will be about perfect.