A good automotive primer helps to bond to the body of the car. after, when you start putting the paint on, the paint adheres to the primer even better. If you wan't a professional looking finish, use a good primer first!
Also Keaton22 Benjamin Moore carries an excellent line of paint called Aura - Self priming to. also has seamless touch-up. 2 coats guarantee.
It would definitely be better to; the primer will help fill in any scratches in your original finish, and to give the surface some "tooth" for the topcoat to adhere to. Just remember, as in any painting job, it's all in the preparation.