And here is another reason why concepts should stay (I'm surprised a Poll wasn't released on this).
Gran Turismo has grown to the point where it not just a game anymore. It is a marketing tool as well. Just look at that Toyota demo with the MTRC and Prius. Had concepts not been in the game, how the hell would anyone know what's coming out, and the performance capabilities thereof? The MTRC had this memorable phrase by our own Solid Lifters:
"Someone forgot...that tandem seating is gay." -Solid Lifters, months ago
But no matter how you look at it, if you can't drive cars that aren't in the game (like the RX-8 concept from GT3), don't you think it would be wise to come up with the concept? I mean, concept cars don't have to just be million-dollar prototypes that go to only auto shows and have nothing to show for their capabilities. I mean, just look at Volkswagen's W12 concept car. I can recall it going through long-distance speed tests, and that car kicked ASS. So why not keep concept cars in GT games, even if they are just... concepts.
With Cadillac bidding for a spot in GT4, it probably won't even matter if the Sixteen makes it. So, marketing is my other reason. If any more, I'll reply in the future granted this one doesn't get locked.