To play devils advocate...
I studied only for the USMLE (and studied my butt off) and did just below average on it, but did very much above average on the COMLEX. I am applying EM this year and as of yet have only gone to DO interviews. I applied to over 50 programs and was told flat out by a few of the more competitive MD programs that I would not be considered due to a low (relatively) USMLE. I don't know if any of this really matters yet as its pretty early on in the interview process and many of these places may not have taken a DO without amazing scores anyway. My gut feeling is that for me, doing a little below average on the USMLE and doing great on the COMLEX has probably hurt me more than if I had only my COMLEX scores. As the programs who understand the COMLEX still just see an OK USMLE.
Also, one thing that has really surprised me is the variation between programs and how they view board scores. One big DO program flat out told me that they don't even consider them at all, while others have claimed to be impressed by my scores. It seems that the DO world really only cares about elective rotations and letters, while the MD world cares much more about board scores. Its been my experience that in the DO match, the only EM programs that will take a student that did not rotate with them are those in less desirable locations. The whole process is frustrating and confusing.
I would tell a DO to take both only if they had enough time to study for it. Understand that if you fail the USMLE, your options will essentially be zero outside of community MD FM programs, and is seems to me that just doing average on it has not helped me at all. Med students are pretty bad at being able to size themselves up and make a realistic assessment of their abilities. They tend to always overestimate themselves. Take a few practice tests and if you are not reliably scoring 225+ I don't see a reason to risk it.
Also, a lot of this depends on the field. From what I have seen, ortho, optho, derm, prs, Irads and ent are essentially closed doors to DOs, thus I would say to hell with it and take both, as a bad score means almost nothing. You will be applying to DO anyway. FM, many IM, peds, pmr and maybe psych are pretty wide open so even a midrange score is not going to hurt you. The difference is in the mildly competitive fields where DO's have been able to match in the past, EM, gas, neuro, rads academic IM and maybe gen surg. (gen surg may be a stretch). A poor score can really hurt you, and an average student may be better off taking only the COMLEX and applying to DO friendly places.
Also I have found from a very small sample size that class rank has little to do with how well you will do on the USMLE.
Just my 2 cents.