One of my fellow designer peeps saw a spot color piece of mine come off the press recently, and swore it looked like a CMYK job. I explained that it was all due to the magic of using spot colors to create quadtone images:
The great thing about duo/tri/quadtones using spot colors is that you can run the graphic elements and typography using full solid colors (instead of a halftone build of CMYK), making those elements sharper, and avoiding the risk of out-of register or unbalanced ink densities making your “solid” colors not look great. At the same time you use halftone separations in the images to build warm, neutral, or cool-toned images that in a lot of cases can look like colors not present in the solid color palette for the project.
In this age of digital printing, which is necessarily process color-based, we can be forgiven for forgetting that once upon a time all printing was spot color-based. It wasn’t until the invention of chromolithography in the 1890s that what we think of as mass-reproducible “full color” printing became something that printers did.