I can see it working great for some stuff but wouldn't you ultimately face the issue with more artistic work that the framing might not be very good if just artificially extending.
It definitely needs to be applied judiciously on a shot-by-shot basis.
There have been quite a few 4:3-to-widescreen conversions that were done using the original film that was actually shot in widescreen and cropped for TV.
Sometimes, the wider shot makes perfect sense. Sometimes, they keep the original cropped one but cut off top/bottom. Sometimes it's a combination of the two. It all depends on what's being framed -- two people in a car usually benefits from cropping (nobody needs the bottom third of the frame occupied by the car's hood), while a close-up on someone's face usually benefits from extending the sides (otherwise it's an uncomfortable mega-close up that cuts off their mouth).
But having the flexibility to extend horizontally gives you the artistic possibilities.
