Having a large number of cars on tracks really adds to the game but the A.I. is so poor it is more like driving through traffic than actually racing.
I would rather have 8 A.I. drivers who actually race you and give you a run for your money than have 16 drone cars.
I just posted this in another thread, but it seems equally valid here (tldr the large fields in GT5 are mostly wasted in a game damaging fashion rather than used to provide a great racing experience):
1: The car must be fast enough to win. There is a point at which no one could possibly win in a given car. Your car must be able to both overtake a whole field of cars rapidly (in order to catch first before he gets way out into nowhere land) and compete/overtake first for the win
2: Your car must be slow enough not to make it a breeze to blow by the competition and take the challenge out of winning.
3: The economy and upgrade system along with no performance ranking for a car make it hard to effectively do this on a regular basis.
4: The loading screens and general clunkiness of getting around GT5 make it undesireably to go back and forth to the tune shop to buy and tinker with buying one part at a time.
I have no interest in getting an overpowered car to blow by the competition, however most of the time this is what happens:
1: Look at likely competition and try to find a car that is near the top of the range. There is no point in buying a car near the bottom that can't win and will cost an arm and a leg to make competitive.
2: Race the car a few times stock in hopes of winning. Sometimes just stock I blow the doors off the competition - sometimes the game just randomly stocks the races with no particularly fast cars. In these cases - situation bad already.
3: If the stock car can't win, I go buy the cheap upgrades (air filter, cat, ecu) and maybe tires. Try again. Chances are now I am blowing by the competition. I COULD go buy just an air filter, then try, then buy JUST a cat and try but as I said, that is quite annoying to do.
4: If I am not competitive with my minor upgrades, I need to buy a bigger upgrade. The problem is these tend to come in 20+hp jumps. So I am likely again to buy some upgrade and now destroy the competition.
It might seem like 10 and 20 hps jumps in power shouldn't make you blow away the competition but due to the way the game is setup, it does. The biggest challenge for winning in most events isn't being competitive with the fastest car, it's being able to pass the pack reliably and catch the first place car before he is way out in the lead. 10hp can be the difference between being able to squeak by an opponent in the turns and not being able to pass them up. When you can't pass the oppoonent, you spend many seconds behind each one waiting for a better spot to pass, when you CAN you suddenly shave many seconds off yoru time becuase you no longer have to wait behind AI cars for the rare easy pass locations and this amplifies (a lot) what 10HP does to your race.
While it is POSSIBLE to find the right car to bring to a race and have a good time, ultimately it's a lot of trouble and the layout of most of the races (read short/no qualifyer) means you don't have a lot of wiggle room in this chase without being a huge time/cr waster.
And if you look way back at my first point 1 - that is the crux of the issue... your car must be potent enough to pass a lot of cars fast because often you will find if you aren't close to first by the 2nd or 3rd lap, he is putting seconds on the pack and there is an almost un recoverable gap he gets ahead of you.
Further more the AI cars in the back of the pack are stupid and slow and if you wait for appropriate times to pass without just brute forcing past, you can easily be laps into the race before you are ahead of the pack in which case we get back to not being able to catch a reasonably matched car with 10 seconds on you already.
So the very layout of the game almost forces you into an overpowered car because the only time you get a good race is when you have a car that is roughly evenly matched with the 1 fastest car in the race. However if you are matched with him, chances are you can't burn through the pack fast enough to catch him early and once he has put 10 seconds on your position, it's almost impossible to make up 10 seconds in an evenly matched card (at least it should be).
It's almost as if the game is built specifically to discourage racing with a well matched car.