I have to agree with the OP, there are only a handful of prize cars that are useful and worth pursuing, especially from lower to mid level when you don't have the quick, fast track, expert level races to grind credits with like the American Championship.
The Zonda and the Minolta come to mind as extremely useful cars. The Bugatti wasn't bad either, useful for grinding certain races like the Turbo Race and getting the speed trophies fairly early.
You also get really good cars if you can achieve golds in the License tests but for most people you need a racing wheel setup to really nail the curves, braking and acceleration right. For those with a DS3 getting a silver, sometimes even a bronze is a challenge with certain parts of the test.
Most of the mind to higher tier endurance prize cars are not bad, but by the time you hit that level you can already get some decent cars with a moderate amount of grinding.