I'll jump in with what will likely (I'm sure) to be a minority opinion here. House paints can and do work great. I've done it for a while, and many in the larger RC airplane world have done likewise. See http://www.modelairplanenews.com/how_to/latex1.asp. *Especially* when it comes to priming, it has worked without problem for me. Sure, it doesn't come in any sort of special effect colors like metallics or pearls, but for basic colors (and again, especially priming), it works fine.
As to finer pigment grinding of hobby paints, I find this to be a common opinion, but I find no objective data to back this up. When sufficiently thinned, I can shoot it through my airbrush and get a very smooth surface. I typically sand all my primer coats, and I've produced results that are as even and smooth as one could ever hope for. I suppose one area where one could find out if the pigments are actually more coarse, is to thin to a level of a translucent wash and see if you can notice a difference. I don't use this technique in my work so I can't offer an opinion as to whether or not its true. But I will claim that you're not going get a sandpaper rough finish, either.
As to whether it will stick or not to various substrates (wood, plastic, metal, etc...), they generally perform very well. The current generation of house paint primers have to contend with all sorts of sub-optimal substrates, (wood, old paint, metal and/or plastic house gutters, etc...) and house painters are not known to be super-particular about getting the substrate to be perfectly pristine. Primers are the first (and often only) line of defense in getting a decent result that won't peel or chip. I would wager that there's at least several generations of more advanced paint chemistry in a top-line house primer than anything we'll ever get from the hobby suppliers.
All that said, I'm not going to sit here and claim there is no downside. First, they're much thicker in stock form than hobby paint. Thinning becomes a sort of trial and error process. I would try a bunch of experiments to get the thinning that works for you. Second, I can't claim to know what sorts of solvent based top coats would be compatible. As a general rule of thumb, one is always safest sticking with similar brands and technology between top and primer coats. As I shoot nearly exclusively with water-based media, I have had zero compatibility problems between house-paint primer bases and any other sort of water-based top-coat. I've also topped with Krylon clear and, on very rare occasion, two-part automotive urethanes, both with no compatibility issues. I'm not sure how an aggressive solvent-based lacquer would work, though.
Here's one brand of house paint primer that I use: Benjamin Moore Fresh Start 100% Acrylic. I shoot it over wood, paper, epoxy, pvc, styrene and metal (many of my models use more than one material). 1 gallon is under $25.