According to an article I found, the acceptance rate is 30%, compared to the College Program which tends to accept more like 70-80%.
For the most competitive ones, including the MIs, it's almost mandatory as most of them require that you've done a CP before. (And they won't consider you if you haven't.)
Very few will take someone without CP experience and those tend to be on the technical end: Engineers, Biologists, Veterinarians, etc.