To me damage would be a nice feature, but it's non essential to the gameplay itself. I try to drive without ever bumping into people, getting on the grass, crashing into the rails so that feature won't be very useful to me. If I want to crash my car, I'd go play Burnout Paradise. XD
On the other hand, it would be nice because it could act as a deterrent to cheaters online who use cheap methods to get better times. If GT implements it, I think they should just add realistic damage, as in you wreck no matter what if you crash at 100kph. I didn't love the damage in Forza and GTR2, for example because you crash going 200mph and then maybe your tracking is little off and your car looks a little less pretty, sometimes a tire pops, but that seems just petty and insignificant. I also want it to be a feature that you can turn off, because I can just imagine my friends throwing down the controller in disgust when they crash and restart for the 20th time in a row.
Anyways, in conclusion, damage isn't important to me, me, being a GT whore, just want to drive a perfect line everytime, I'm not interested in seeing my car demolished at the finish line by the AI, to me, it's going to make no difference whether it has damage or not, so I will still buy it, heck, I'll buy 2!
Edit: I didn't read the thread... and all my thoughts have already been posted. XD