It is realy not that complex. You dont streatch 16:9 material.
With a 21:9 you can get a bigger screen for movies without getting a huge TV set, consider a 21:9 to be a streached TV set - a 56" 21:9 is a streached 42" 16:9. Instead of buying a normal 42" you buy a 56" 21:9.
The material in 16:9 aspect ratio is watched in same size on both TV sets - but films much bigger on the 21:9 ( corresponds to a 61" 16:9 TV set ).
If you have the room, you could ofcause buy a 62" 16:9, but if you have the room for this, then why not buy a 80" 21:9 which will seem smaller than the 62" 16:9 ---- and so on.
One important issue is the movie experience, for some reason the cinematic experience is much higher when the movie fits the screen. If a Philips, the Ambilight fits to the movie.
This is ofcause highly subjective matter, there is no right answer.