Good point, and I have been out of the business for a while, and there are probably some advances, but generally a primer is an ad on sale. Granted, they don't usually put as much, if any, mildicide (for example) in primer, but that just lowers the price, and makes room for more clay, price wise. Generally you'll find that they don't put any more 'good' stuff in the primers, they just lower the price. Try it yourself, take a primer and a topcoat of the same vehicle and paint them both on some Formica or aluminum. After they cure (not just dry), you'll find the primer scrapes off just as easily as the paint.
Now granted, some primers have their purpose, but rarely is 'adhesion' one of them. For instance, down in South Florida, SOP for re paints on stucco call for pressure cleaning, then priming (called sealing down there) with a clear acrylic sealer. This is simply a watered down acrylic paint with 'no' pigment. It has no pigment, and is thinned down, so it is able to 'absorb' and 'encapsulate' the chalk that is inevitably left over after pressure cleaning, so that the chalk will be much less likely to cause an adhesion problem with the topcoat.
It is more of a 'sealer' than a 'primer', but it can be considered a primer coat. Now this product 'technically' adheres better than a topcoat, but then, anything without pigment will stick better than the same mix 'with' pigment. That principle holds true with topcoats too, the less the pigment to resin ratio, the better the stick. A gloss acrylic will adhere much more than an acrylic flat, etc.
But no, as far as 'properties' go, if your surface does not need to be altered finish wise by the 'filling' properties of an undercoater, then there is no reason to use anything but one or two coats of topcoat.
