I like collecting cars just so I can drive them and experience the differences. After buying some good racing and road cars I decided to buy every car under 50K credits, this was fine until the Japanese cars, lots and lots of similar Nissans, etc, made it hard work.
However I do like having a completely stock car and if I do need to upgrade a car for the time trails I tend to buy another to modify it to the race spec.
I do agree though that the Stockyard is really annoying, why can't we just have one garage for our cars, it would make life so much easier.
Got to say though that I didn't like the UCD in GT5, I felt that I had to buy every car there asap or miss out, and because of this I didn't really test all the cars. This is what GT6 is better at, you feel like you own the cars and want to test them rather than just buy everything in the UCD and then not always have the time to test them.