I hope your profile isn't updated because if you are less than 12% complete there is a lot to do before I think you have seen enough to really get into the game.
As for some of the prize cars being useless it seems to me that people complain about one of two things (sometimes both, whiners) that the prize cars are too valuable or they are too useless. I think the antique cars have a place in this game considering that after GT2 came out KY said that he wanted Gran Turismo to be an encyclopedia of cars. And if you think they are useless or that no one would go out and whip them around Midfield then you should read
this thread. For some of us it creates a whole new challenge. It is KY's vision and not yours. Your only choice is to buy or not buy.
Perhaps you aren't being creative enough. I constantly hear people complain about cars like the minivans, SUVs, trucks, and wagons. Wagons have been in GT since GT1 and the others were only a matter of time. My friend and I get together and have minvan races, wagon races, trucks races, etc. If it is odd and funny then we do it. If you really want a challenge try racing your friends in a less than 100hp race. Those cars all seem pointless too, but when you are fighting for every ounce of speed you can get it suddenly becomes stressful.
I love how you can list a few prize cars and complain, but what about the Toyota 88C-V or the Formula Gran Turismo or the Audi R8 or the Sauber Mercedes C9 or the tons of other fast and/or rare cars? If you aren't in the endurance races yet and haven't unlocked the extreme hall yet then it is my opinion that you haven't done enough to see what the game truly offers.
As for my complaints, the laundry list of bugs are annoying but don't ruin the game and the only prize car complaints I have is why I have to win the Nike 2022 three times and why is the Model T the hardest car to get? I want it. "It will be mine, oh yes, it will be mine."
And I will agree the AI is horrible, but those complaining about B-Spec may find he gets better when you train him, which means racing on multiple tracks and not just blowing everyone away with power. My B-Spec driver has about 7000 points and can handle most tracks at 4 and some at 5. He only has passing troubles on narrow tracks like Paris or Hong Kong.
I was quickly bored with GT3 but GT4 has addicted me in a way no other game has. You just need to get over that slow moving early on hump. Reach your 25% and open up the Extreme Events and then you will be into the meat of the game.
As for motorcycles in GT: They aren't cars in my opinon, but if they put them in I will race them too and just have another angle to the game. It might be fun to slam into teh wall at 200mph just to bounce in the air and land on my wheels.