Comparing a flight simulator with no crash mode to a racing game where you are driving around a track with other cars and real obstacles to run into isn't really a valid comparison. Having played GT3 and GT4 as well as Forza 1 and 2, I can tell you that having simulated damage makes a TON of difference in how the game is played. For instance if I am trailing a CPU (or even player controlled) car going into a sharp turn in GT then there's no reason for me not to just T-bone the lead car and put it into the wall as I make my turn, but if I do that in Forza I damage my car, jack up the fender, damage the alignment and steering, etc. degrading the performance of my car and my ability to race, so I have to use better technique to execute the pass without damaging my car. I can still rub or bump the other guy, but not just flat out crush him. Also, in Forza 2 when running in manual if you're going flat out down a straight away in fifth gear and just drop to third without breaking and letting off the accelerator first, you damage your engine (slightly, but keep doing it and you'll blow the thing), instead of just dropping two gears and redlining.
I really hope that GT5 gets real time damage, either up front or as an add-on, and I hope that it's damage that's not just cosmetic, but affects performance as well. It's a real gap in the simulation aspect of a series that prides itself on the level of detail and realism in its simulations.