You make a very good point. I'm a sole copy center operator in an in-house copy shop, so my experience is definitely different than those in the competitive world.
I've often thought of starting a company that goes around teaching pre-press - there are so many variables that are hard to catch. For instance, if a white logo coming out of Illustrator and imported into a document, reversing out of, say, black, doesn't print as a reverse, many times it's because in Illustrator the logo is set to overprint. How does one figure that out on their own? Plus the naming of colors, when to use rich black and when not to, etc.
Cathie