It comes down to a personal decision, really - as to whether you go full 2.35 or stick with 16x9
That said, my personal choice is 16x9 for the den, and 2.35 for the cinema - here's why...
Almost all movie releases are in 2.35 (or 2.4 etc) - and most releases to home video are indeed letterboxed to fit inside a 16x9 space, so the presentation area is still 2.35
In my den, I have an 86" LG 4K TV, which is perfect for mixed content from cable, AppleTV, Bluray, etc - so a letterboxed 2.35 movie is still large enough to be appreciated on this TV
But as a purist, I wanted my main theater setup to show the full experience as intended, so my theater screens are 133" across, 2.35 AR, and I simply "zoom" the projector, to fill that 2.35 space with the letterboxed content, typically from Bluray, or whichever source. I have separate screens (both 2.35) for 2D (black diamond II 1.4 gain) and 3D (black diamond 3D 2.7 silver screen)
This is shot with two JVC NX7 projectors - only one pj is used in regular 2D presentations, but both projectors are utilized when viewing 3D content (linear polarizing filters on each pj for left and right channels)
So as you can tell - my setup is probably a little more over the top than most - however, if you have a separate den for your main TV usage, and a separate setup for movie nights, its just a shame not to go for a 2.35 AR screen for the movie setup - a missed opportunity, in my opinion
Don't forget, you can still zoom down and show your regular 16x9 content, centered in the 2.35 screen space
Anyway - that's my 2 cents on the question raised by this thread
CHeers!