I agree that being a runner can be a great help when coaching, especilly at the H.S. level when supervision and such are also a part of the job.
But, I can also think of some good runners who are terrible coaches but they think they are God's gift to the running world. I think someone said before that to be a good coach it can't be about you and what you may have done in the past, but what you can get your athlete's to do now.