Seems like PD blurred the line between Arcade and GT/career mode themselves. In GT5, arcade feels superior to GT/career mode in every way. Likely because the only difference seems to be Arcade lets the player decide what cars, tracks, and ai. Which is a lot more rewarding than racing Like the Wind or Sunday cup over and over.
For me, I buy cars in GT/career because I have to. While the fun comes from online & arcade races. Career mode is just kind of bleh after you do it once, but you have to do it get new cars. PD needs to rethink something somewhere. I mean, I love GT5 for the driving/racing experience, but christ could the rest of it use some help.