Took both USMLE's and both COMLEX. I don't think it hurts you all that bad at average programs but it can seriously help you at top programs. I was flat out told when I interviewed at some top places that my USMLE scores had opened the door for me. I don't think it closes too many doors by not taking it, but it sure can help...big time. PD's seem to look more favorably on high USMLE scores than high COMLEX scores. Its just one of those things they can use to strengthen their justification for wanting you against a stellar MD student, if they have something to compare apples to apples. I recommend it only for DO students who scored > 600 on COMLEX. I believe anyone who scores less than this is highly unlikely to do well enough on USMLE for it to help them. You are just wasting your money. But >600, and especially >700, my experience tells me you have a good chance of rocking the USMLE (especially step II) and you should do it.
If you are average though across the board, you just need to apply to lots of places and don't hold out for places like Carolinas or Vanderbilt. Be happy with interviewing anyplace and just play the statistical odds by doing lots of applications and interviews.
FYI, applied only allopathic EM, >40 programs, 75% interviews offered, went on 15 total, and got my 3rd choice. My top 2 choices were probably well out of my league to begin with and I know I am much happier being at my 3rd now that I am headed there. I'm now a Hawkeye!!