There are a couple of problems with 4:3 screens.
1. Source quality
Most 4:3 sources (DBS satellite, bad cable, VHS tapes) are extremely low quality. Most widescreen sources (DVD, HD) are high quality.
If you use a 4:3 screen small enough to keep the bad 4:3 sources watchable the good wide screen sources are too small. If you select the screen for the better wide screen sources the soft picture and compression artifacts from bad 4:3 sources become objectionable.
A wider screen is smaller for your bad 4:3 sources, and bigger for high quality wide screen - the best of both worlds. And the 4:3 image is still a lot bigger than an RPTV on "modest" front projection setups (81" diagonal on an 87x49" 100" diagonal 16:9 screen).
2. Space limits from the room and speaker placement
Disregarding the above if you wanted an 8' wide screen for good scope movie performance a 4:3 screen would be 6' high versus 40" for a 2.35:1 screen. You'd have a hard time getting a good center channel placement on the 4:3 screen without going to a perforated setup.