I've read the FAQ on primer but...

My priming is very hit-and-miss; if I have some badly colored plastic (like an old Matchbox kit with dark green and white), I'll prime. If it's a dark plastic to which I'm applying a lighter color (tans over dark grey German tank kits), I'll also prime. If it's a USN jet, I won't as most kits are molded in light grey. I look at it as going for truer color coats and not smoothing the finish though a car modeler will use the primer as a smooth undercoat for the color coats. It all depends on your subjects and preferences. FWIW, I prefer Floquil's lacquer primer, the grey stuff from their railroad line of paints...
Frank Kranick

Pretty much the same here but as for cars I often use a primer underneath for colour variations. Most colours will change their appearance based on what they're laying over. F'rinstance I have a ModelMaster red metallic that I've laid down over white, tan and green undercoats and every one of those cars looks like a different metallic red. One of my car buddies complains that he's tired of painting cars different combos of black, red and white. I guess he'll stay that way because he lacks a sense of 'adventure' where it comes to paint.
Bill Banaszak, MFE

Well, I'm a car modeler and I prime because I like to use the paint from MCW and it is lacquer based automotive paint, would do a nasty job on the plastic....so I use their primer that is compatible with the lacquer and the plastic kit. Priming also helps with the customizing I do as I like Squadron Green filler, but it's a bit hard to hide unless you prime first. And as Frank said, it helps with getting a very smooth paint job. Mike

I have found that water-alcohol based acrylic paints don't "take" very well on clean plastic. I have found that a primer with some bite to it, I use floquil gray primer, gives a surface which an acrylic color coat will adhere to much better. My favorite primer for many years was Pactra flat gray or flat white enamel thinned with lacquer thinner, which had just enough chemical "bite" to adhere very well. I do miss the Pactra line.
Bill Shuey