I use two coats of paint on every exterior job. Primer isn't paint and can't be used in place of paint. Modern acrylic paint is designed to have a certain thickness when dry (mil thickness), this is usually very thick. The best way to achieve this is to apply two separate coats with ample dry time between each coat of paint.
As to the wax analogy; Yes the top coat does all the work. But, what if it should fail? Eventually the top coat will oxidize and the job of protecting your home now falls to the first coat. I would rather have paint protecting your house than primer.
Just my two cents worth (after taxes and inflation).