I've brushed some straight from the tube, years ago, and gotten fantastic adhesion—but on a textured surface. I've also used them on polyester and epoxy surfaces with good results.
But—and this is a big caveat—except for armor, and even then, plastic model surfaces tend to be very smooth, and using a paint not formulated for a styrene substrate may not work well at all, as Scott points out.
And color matching becomes highly problematic—you may waste all your savings in materials by having to buy more colors just to mix the ones you need. And you will spend a lot of time, and waste a lot of paint, trying to mix military colors from artists colors.
Part of what you are paying for in specialized model paints is savings in time and effort.
If, on the other hand, you are doing figures, artists of craft paints over a good rattlecan automotive primer should work well.
Yes, you can do it. But a good bottle of paint, properly cared for, can last decades. In my opinion, the best way to economize with paint is to buy good quality and take care of it.