I can't speak for anyone else, but I have sprayed plenty of cars and music gear (using a compressor and guns), and my thinking is this : On a car, one is somewhat limited in the viewing angles relative to light sources, and guide coats provide a great way to see the high and low points. Guitars can be comfortably manouvered into almost any viewing position under a work light, and light scuffs with fine sandpaper is enough to see any discrepancies. Combined with block sanding, great finishes can be achieved without guide coats.
Also the much larger surface areas involved with cars will show up discrepanices more noticeably, from small lows and highs to much larger 'waves', whereas the average guitar has only small areas once the hardware is fitted ... the largest single flat area on most guitars is usually the back.
The majority of guitars shouldn't be relying on levelling thick layers of primer anyway, most of the levelling should be attended to on the wood itself, and to some degree on sealers used before the primer. Too much thickness in primer coats, esp the filling-types, can cause problems later on both guitars and cars ('crowsfeet' is a typical symptom on cars, as the excessive builds below the colour coats continues to shrink after the top coats have been applied).
That's not to say a guide coat may not be of help sometimes, but i think it's a lot less necessary on guitars, and thousands of very high-quality guitar finishes may attest to that. Just my undeucated (but somewhat experienced) opinion.
