A good primer coat is essential for cars or airplanes, (full size,) I doubt that any pro shop would try to turn out a quality job without priming, unless painting over a good paint job already in place, properly sanded and prepped.
Well there are big differences between a real aircraft or auto and our plastic model kits:
1) the surfaces involved, and
2) the paints or lacquers used on them
3) the conditions these items will operate in and what is expected of the paint under those conditions.