I've heard that GT5 content will be available as a download or reduced price for Prologue owners, so I wouldn't be worried about that aspect...especially since the full version probably won't be out this year.
The Prologue is a fairly full game now with car tuning and extra cars in it. It's very well priced at $40.
As for me, I'm going to rent it and assume my save will carry over to GT5. I've never played a GT game and don't buy many games anyway, so to me this is the perfect rental.
They would never do that they would lose money they make you pay $60 to buy the full game. GT 5 would have like 4 times the tracks with 24 and 4 times the cars with around 300.
They could lose fans by charging again for the full version.
Interviews with the developers were the sources for the possiblity of Prologue being a base for future DLC content (such as damage) to be added. I think it's a real possiblity.