I do
In my opinion, only mechanical damage needs to be implemented in a driving simulator. Visual damage is just the physical representation of the consequences of racing.
If I crash into a barrier at 200 MPH, it doesn't matter to me whether or not I get to "see" the damage I've caused. What matters is how that crash affects the overall performance of the car.
Don't get me wrong, I wouldn't mind having visual damage in GT5. But as Jetboy said, the problem is that it would come at the expense of other things that are far more important to a realistic driving experience. Like a smooth framerate, or having 16 cars on track at once, or the physics. If visual damage was included, the cars would be less graphically detailed. Many of you have said that you are willing to take such a tradeoff, that it is more important than any of these features. I understand your point - since day one, Gran Turismo has been marketed as "the real driving simulator," and therefore it must be as realistic as possible. I, on the other hand, am not willing to make that tradeoff.
This is a game, not real life. I believe that, as a game, it should have all of the benefits of real racing without some of the drawbacks. As TopGearFan08 said, I don't take joy in seeing my favorite car destroyed, even in the virtual world.