Here's how I looked at it: Who better than a D.O. to say whether you have what it takes to complete the education and training. Furthermore, who better than an alumni of the schools to which you are applying to say whether you are likely to succeed there. I don't think (don't know) if any of the schools *require* a letter from a D.O., but I know that some of them STRONGLY recommend at least one. If you don't already have a letter or someone to write one, find a D.O. to shadow for a couple days and ask for one. There is a list somewhere on the AOA site of D.O.s who want to be mentors, or just start cold calling out of the phone book.
Good Luck!