As much as I love seeing a woman's face as she experiences pleasure, so much more can be done while you're on your back. The drawback is that women sometimes feel like their role here is to just lie back and enjoy. They feel more comfortable when you’re on top doing the work.
It's also a good go-to position when you are working with inexperienced girls. When I have that control, I’m more capable of making myself orgasm, and I'm all for orgasms. It's the position where I feel most in control of what is going on, probably partially due to my familiarity with it since it’s over-used.
Though I think missionary is over-used, it serves several good purposes.
