I used to think that way also, but have since changed my mind. Some of the top coaches in the industry get coached by someone else if they are still competing. The reason is not because they don't understand how to diet, or how to train. It's because it is very difficult to be objective when looking at your own physique. Especially in the last few weeks, when your eyes play tricks on you, and you start second guessing yourself. Not sure if you are lean enough, or full enough, or maybe over dieted etc... Those that haven't competed, will not fully understand what it's like, when you are contest ready. So that's when a second pair of eyes comes in, and you just turn off your own brain, trust the person and do what you are told.
That being said, I'm still planning on competing this year without a coach. Reason being, is that the coaches I respect, I don't think I can afford and I doubt they would actually pay attention to a nobody like me. The more affordable coaches I don't fully trust. But I do have know personally a couple fairly good coaches that work with IFBB pros and will rely on them just to get quick feedback on my physique, and how I'm looking. And I'm sure even the top coaches who don't use a coach when competing (like John Meadows) have some trusted eyes in their circle they can rely on for feedback also.