That's missing the point: in a garage of many cars (I think I'm hovering around 1100), it's very hard to find cars that qualify for "stock" races. You can colour-code, you could remember which are which (I tend to remember which cars are suited for which purposes once I am only viewing one particular manufacturer), or you could use the Favourites for only stock cars. But there's no easy way for the user to organize their garage how they want, nor filter for the "stock" limitations. Which we know PD is capable of implementing, since your choices are limited to eligible cars only when entering the garage from a race's info screen.
FM4 allows a user to create folders in their garage, and put whatever cars they want in it. It lets me organize my garage specifically how I want, and it'd be very handy if we could create extra tabs in GT5 (similar to the Favourites one) to keep our garages more efficiently ordered, especially taking into account GT5's Pokemon-style gotta-catch-em-all UCD. It'd certainly see plenty of use from me 👍