I doubt those who can't find value in Prologue will find it in GT5. If racing doesn't captivate your interest now, what about the full game will? I assure you painting and modifying cars is no major selling point for true fans of the series, and fans of racing in general.
I've constantly been in competition in one form or another, and racing in a league, or having TT competitions is one sure way to extend the enjoyment of the game.
It's racing... the car and the track are merely the brush and the canvas. They matter little. What I measure the value of prologue on is the time I've spent playing it, and I have been avidly racing since the North American launch. In a one year span, I have never played a GT game as much as I have Prologue. Why? Because racing against real live humans turns the fun factor to 11. Everything else is peripheral. What good are private rooms, if you have no racing friends? What good are dozens of tracks when your driving skills are lagging behind and the majority of people are leaving you in the dust? What good are all the tuner parts in the world, if you can't setup a car worth beans?
Could Prologue be improved by adding more features and fuctions? sure, but then it wouldn't be much of a 'Prologue' anymore, and we would have another not quite finished/ rushed out product. The full game will come when ever it comes, because that's how long it takes to make it. In the mean time, thousands of people are still enjoying Prologue because they can race, online with real people, 24 hrs a day, in HD, with some of the worlds most recognizable cars. How many games can say that right now.
I've spent my money on some overhyped garbage games, and Prologue ain't one of them. Since when does the quality of the game determine the price anyways (time spent sitting on the shelf is the only thing that brings a game price down) I paid the same price for MGS4, MLB The Show, Oblivion and GTA (good games, worth the money) as I did for HAWX, Assassin's Creed, Motorstorm and Socom Confrontation (not worth the money). Prologue has destroyed them all for play time, and it came in 30% cheaper! That's Value!
Also, did you really expect to find support for your opinion at a GT fan site?