After a splendid undercoating with Bringiton's Zinnser B-I-N shellac-based primer -- and what, after all, are a few brain cells lost to the fumes when what's at stake is a really smooth surface -- I laid down the first coat of color today, and had an insight of a sort. (This whole process is hard for me, since my father taught me how to paint when I was a kid, and the last time I did any real painting what my father taught me was still true; but not so now.)
Work with me on this:
Oil-based paint is like oil painting.
Water-based paint (latex) is like water-color.
Painting with oil is all about pushing the paint into the surface (canvas or wall) so that it bonds; it will then smooth itself out. It's possibly, and because of labor, desirable, to get the final color done in one pass over the primer.
Painting with latex is like water-color: The color is built up with a series of thin washes, one over the other. It's not about bonding with a surface at all. If there any brush-strokes show, you've laid it on too thick. Ditto rollers: In fact, I'm evolving toward a "dry roller" technique that puts the absolute minimum film down. Different from oil, again, where the object is to get a maximum down.
So, the trade-off, leaving aside the environment, is oil, fewer coats but nasty clean-up; latex, more coats but much better cleanup -- and probably, since the applicator of choice for latex seems to be not the brush, but the roller (or even, shudder, the spray gun), one coat of latex goes on faster than one coat of oil.
Don't know, I'm not a professional...