Dress for Success
People judge with their eyes. Like it or not, human nature is to make a first-impression judgement of someone, and our best sense is our sight. In professional audio, the average person is not going to hear the difference you are making by being there. In a lot of professions, it's likely your task is so specialized that the average bear can't tell the difference between you doing it or the volunteer getting it done. People can't hear the difference between a good mix and a bad mix, they can only notice if a mic doesn't come on. So, as long as you're making all the right mics come on at the right times, they're happy.
So then, how will they ever notice if you truly are a professional, and truly worth the extra effort to call you in, instead of putting a last-minute volunteer on the board?