[teebrz]

There are directors who have scored one or many of their own films, such as John Carpenter, Robert Rodriguez, and Clint Eastwood. It's true that excepting the ones who have a strong music background director-composers usually produce somewhat simpler and critically not-acclaimed scores but they also often end up working well. Look at the Halloween theme; it's a very simple melody but it somehow captured the mood of the film and people's imaginations.

I agree with your general point though, a collaboration between the "user" and "expert" will generally produce something superior; but sometimes there can even be a little bit of a liability having the other party around.

The school teacher thing is a poor example though - that's just marketing trying to make an appeal to authority. Something like if a teacher had designed a desk or chair would be better because their job gives them insight into that as opposed to say, a mail carrier.