Sometimes, especially with 1-color paint schemes (Russian tanks come to mind here), kits come molded in the color you're going to paint them in anyway. Could you get away with skipping the "painting" part of the finishing process, and move on to the clear coat (for decals)? Or do you need the paint for something later on, like weathering?

my friend, I always use a base coat so it does not matter what colour the plastic is molded. It would not look like the work of a genuine modeler, but rather that of a child if it was left unpainted. The other thing of course is that a coat of paint would cover any excess glue stains and other marks as well

The unpainted plastic will give the model a waxy or soap-like appearance. While it is OK for a beginner or child, most modelers will apply at least a base coat. The tank will look more realistic and give the subsequent weathering paints something to bite into.

Now I have seen car modelers who do not paint the body color of cars. These people tend to put massive amounts of time into plastic polish to give the plastic body a mirror-like finish. Other items on the car body like the cowling, chrome trim, etc. are still painted and detailed.

I would think even if what Sabot says isnt an issue
(AND IT IS )
you would need the base coat to hide imperfcetions of fit, gluing,
ejection molding pins, etc etc

I dont know why model companys bother doing this.
No one who is hardcore will accept the plastic without paint
(as this thread proves)
I could use a contrasting color to help me see the thin or missed areas when I'm applying the base coat. Without the contrast, it is tougher to tell if you missed a spot.

No matter what you do. unpainted plastic will always look like unpainted plastic. Most of us go beyond just adding decals with various levels of weathering and detailing. If you're buiulding strictly for yourself, feel free not to paint. But if you're going to share your work with others, you will really want to paint your models.