I think what you are asking is do you need to prime a kit prior to painting it or just paint it without.
The reason you use primer on plastic is not primarily to give finish paint something to hold on to like you would when painting metal on a vehicle, although it does do that in a way. The reason you prime is to show you what the kit looks like and shows defects and areas that need attending to before you put paint on. It is a last way to see the blemishes and fix them before using your finish paint.
Using primer or not is your choice. I use it for the resons I have stated but there are many who get great results without it. I feel safer with it and have nearly always found flaws I didn't see prior to primer that I can then correct.