Spot colours (normally Pantone references embedded in the file) can be pretty expensive, and obviously scales with the number of colours you spec. The printer needs to either order or mix your inks, then run it thru a single colour print process or something automated like a risograph.
CMYK is ok when printing on white stock and has the benefit of being easy to print. It is washed out on screen, but it can look pretty nice when it has the benefit of light + bright white paper behind it.
