Grendels said:
Here is a good article about primers are whether you really need them:
http://www.scalemodelguide.com/guide/painting/all-about-primers/
They do serve a purpose, but if you are using model paint designed for a plastic model, on a plastic model, then a primer should not matter.
Some info in that article is simply not true, especially when he goes on about the color of the primer doesn't matter as it doesn't effect the end color. Simply not true. If you take 3 plastic spoons, prime one white, one grey and one black, and then put the same color over all 3, you will end up looking at 3 different hues of the same color. Mind you this is more used in the model car world, and not so much elsewhere, but the fact remains that the color of the primer will effect the color of the top coat.
While I am sure great finishes can be achieved without a primer, most acrylic model paints will adhere much better to a primer than the bare plastic. If you are working with an enamel or lacquer, you may not need the primer as much, however an overall primer coat gives you an even playing field to start with if you have brass, resin, or even just putty filling seams on a kit. Gives an overall even canvas to start your color coats with.