If you paint with enamels, priming is not needed from the paint adhesion perspective. If you add a lot of scratch built details and PE that are vastly different color than the underlying model, i.e. white plastic details, brass PE, on a dark green tank, you may find it beneficial to spary the thing with a medium gray just to allow your base color coat an easier coverage.
If you paint with acrylics, you may find that priming will help with the paint adhesion. Although I do use acrylics for some things, I rarely am painting bare plastic, although in my limited experience with it, I found that Tamiya Acrylics seemed to stick pretty well without priming or any other special surface prep.
Andy