Priming is always a good idea to even out the overall finish of the final project. If you paint the final coat without it you can have parts with weird finishes from the metal to the plastic. Primer is essential to protect the metal parts from corrosion and adds tooth for the paint scheme later. Plus you can sand, putty, fill holes, add more parts and then prime again to check your finish before the final coat. I included my M4A1 in progress as an example. I just added the metal schumo bits and am about to add putty for the welds. once complete I add more primer and finish up. This is just my opinion of course.