Essentially I agree with everyone... kinda.
However, I am too anal to just assume my COMLEX score will potentially open all of the doors I want it to. I wish I was at a point in my education to give you some first-hand experience from the interview trail, but that's a year down the road yet. All I know is I have busted my butt to get where I am number-wise and I would be very disappointed in myself if not taking the USMLE prevented me from having a strong shot at my residency of choice.
You have to admit, having a USMLE score to directly compare us to allopathic students, at least number-wise, could be a good thing. Of course if you bomb the USMLE that theory goes straight out the door.
Also, as I do not know how much emphasis is placed on the two digit scores, I certainly do not want my 97 on the COMLEX equated to a 97 on the USMLE, as they are quite different scores. (COMLEX 2.75 SD above mean, USMLE <1 SD above the mean - ballpark)
Overall, I see it as another way of demonstrating to prospective programs your academic achievement... Again, that is if you do well. If you are cool with limiting yourself to Osteopathic programs and the Allopathic programs that accept the COMLEX, I say don't take the USMLE and save yourself the expense and stress.
BTW - I think the reason some people may say take step II only, is that the test is more clinical and less book-worm material like step I. Therefore, except for the OPP stuff, the tests should be about identical in terms of content. This is not the case with step I, though, as I think we get less embryo, biochem, and biostats to name a few.
I think you will be fine either way. I did it because I wanted as many options as possible.
Regards