There are plenty of examples of guys in the current era who've found success after being journeymen or having a rough period and credit their pitching coach with fixing their mechanics/teaching them a new pitch/ changing the way they attack hitters.
Guys like Roy Halladay, Cliff Lee, Joel Pineiro, Kyle Lohse, Chris Carpenter (notice a trend for Dave Duncan?) among others are all guys who were heavily influenced at one point or another by the teachings of their pitching coach.
I don't understand the inclination to try to dismiss the usefulness or acumen of people who do this for a living. A lot of fans are very knowledgeable about the game, but watching and experiencing it are different things. If pitching coaches were as useless as some believe them to be, they'd be out of a job.