The Catbus has tiny lil balls. This is the kind of attention to detail that wins awards for animation, damn it.
Also, the Catbus having tiny lil balls suggests that the Catbus is able to procreate with a female Catbus. OR a bus. OR a cat.
Probably the biggest plus of riding the Catbus is that the strong smell of cat pee can be assumed to be from the bus itself as opposed to another passenger.