While you don't need a primer when using the Tamiya spray cans, it is recommended, and will give you a better finish in the end, especially if you have any dissimilar parts (IE, plastic, resin, metal, photoetch, etc), as it will give you an even coat over those items to start your color coats from, as well will give a more durable surface for your color coats to stick to. Compare it to GM, Ford, Chrysler, etc. when they paint a real car....it always has a primer coat.
As well if you have alot of putty work, the primer will even out the surface from your plastic to putty, something using just paint won't do. I always recommend to my customers to use a primer, not because it means more sales, but because the final product will look that much better, and the customer is happier that way.