Well, I'd say only an idiot would decide to buy a game or not through review scores alone, no matter how many.
I really believe PD are shooting themselves in the foot by not releasing a demo. In this generation, you don't have to rely in questionable reviews and scores anymore. Download a demo and experience the game by yourself. Casual gamers have gotten used to downloading demos to help them decide if they'll buy a game. And believe it or not, casual gamers are the great majority of GT buyers.
Do you realize just how few games actually get a demo on PSN? I think you would be suprised by just how few there are. For example the game I have put the most hours into on PS3 is 'Fallout 3'.......good thing I didn't wait for a demo before I decided I wanted to buy it based on the 90/100 score it got on Metacritic or I would still be waiting.
Why not trust a large average of review scores.....they after all get to review the entire game. How much of the game do you think a PSN demo would contain. 1 track and 5 or 6 cars......Could you really form as good an impression of the game from that as a large number of reviewers could from trying the full game.
I seriously doubt it.
As others have said.......if you simply HAVE to play the game before you decide to buy there are plenty of ways to rent it when it is released (just as I am doing with F1 2010 to see if I think it's worth buying).......or there are plenty of stores you can try the demo in......or you could wait for a good average of review stores........or you can buy the game and if you dont like it return it for another......