Paint is complicated stuff, and I don't have the scientific knowledge to explain just exactly how it works. Somebody who knows more about it than I do explained the difference between acrylic and solvent-based paint (e.g., enamel) this way: solvent-based paint dries from the inside out, whereas the surface of acrylic paint dries first. That certainly is consistent with my experience.
My primary interest is sailing ships and I do about 95% of my painting with a brush, so my experiences may be a little different than those of other modelers. (I'm sure T J Ace's points about problems with masking are well-taken, but I rarely have occasion to use masking tape so I can't really comment on them.) I'm a big fan of acrylics, my favorite brand being PolyScale. My biggest complaint with it is that it dries a little too fast for my taste.
I've been using water-based hobby paints since PolyScale's latex predecessor, PolyS, came on the market about thirty-five years ago. (I'm not sure what the technical distinction between latex and acrylic is, but according to the company that's the difference between PolyS and PolyScale.) I became a fan when I realized that my model-building sessions were lasting twice and three times as long as they did when I was using enamels. (Reason: I wasn't getting headaches any more.) Initially there was some doubt about dried water-based paint's ability to stick if it got wet. To find out, I painted a 1/700 tugboat with PolyS, let it dry for a week, and submerged it in a jar of water. A year later the model was still under water, and the paint showed no sign of coming loose.
I certainly agree that the bond between acrylic paint and styrene isn't as strong as the one created by enamel (or lacquer) - initially. I think that's because, though the acrylic may seem quite dry to the touch within a few minutes, it takes much longer than that to cure. But I've got a couple of models that I painted with acrylics twenty and twenty-five years ago, respectively. The paintwork looks as good as new - and it's really, really stuck on there. Again, I'm not a chemist or a physicist, so I don't have the vocabulary to describe the scientific process that takes place at the interface between acrylic and plastic. The acrylic certainly doesn't dissolve the surface of the plastic, like lacquer can do. But after a while - maybe as long as a week - a bond
does take place, and is remarkably durable.
As for priming plastic - as a general rule I've never found it necessary. It's a good idea to wash the parts in soap and water, to remove any mold release, but in all probability any paint used as a primer won't actually stick better than the finish coat. The fewer layers of paint go on a plastic model, the better. These paints are amazingly thin, but each additional layer does, by definition, obscure the underlying detail a little bit.
I have run into some situations where an undercoat was desirable - not to make the finish coat stick better, but to help camouflage the color of the underlying plastic. In another thread we've been discussing vapochilled's Heller H.M.S.
Victory. That enormous, beautifully-detailed kit includes over a hundred gun carriages, which are molded in black plastic and need to be painted yellow ochre. If I were taking on that project I'd probably spray the carriages with a light grey first, then apply the yellow with a brush. (I'd probably use enamel for the undercoat, to eliminate any chance that the acrylic finish coat would soften it up.) Modern hobby paints are wonderful, but asking yellow to cover black in one coat is asking a lot - and asking me to apply more than one coat to a hundred gun carriages is asking more.
On the other hand, the barrels of those guns are molded in black, and need to be painted black. I can see no reason on earth to paint them grey first.
That's my two cents' worth.