I think some people are under the mistaken impression that paint and primer in one makes it a one coat paint job and I don't think that's the case. Even though the instructions don't say it on alot of these products, I find you still need 2 complete coats. Paint and Primer in one just means you can use the first layer of the paint as a primer coat and the second layer as a top coat. When you do it that way they are OK but nothing spectacular.
You can probably call it a marketing ploy because some of the manufacturers don't really explain this, and try to convince everybody all you need to do is slap one coat of this stuff on fresh drywall and you are good to go.
I'm not aware of any manufacturer instruction that you can use one coat. All P&P applications I've seen require 2 coats to be warranted. Aura specifically mentions that 1 coat is possible for repaints, but over bare drywall they require 2.
It might well work (I've put SuperPaint flat over bare drywall in 1 coat and had it come out good), but it's not warranted that way.
eharri3- your point is right on. Most high quality interior products are self priming, although a certain consumer brand has bastardized the meaning. This is what gives consumerers the impression that they do not have to perform surface prep or clean the surface being painted.
Consumers might have that impression, but if so they didn't read the directions.