I think the quality of a letter is more important than the tittle of the writer. In the case of a volunteer coordinator verus an md, then I'm sure the adcoms understand that the coordinator really was your boss in this situation.
I had a coordinator write me one and I was very grateful that she did - I had spent about a year and a half working in her department and even tho I worked nights and she was on days, and we never saw each other really, in the end she was my 'manager' and was responsible for me being there in the department.
I think that she gathered people's impressions of me and my work and used that to write her review. So really, it was a very comprehensive evaluation, I would imagine.
The md's in the department were WAY too busy to be bothered with us volunteers anyway so it all worked out in the end ... and yes, I would definitely consider this letter to be one of my major evaluations (we have Committee Letters).
You should be fine.
