I feel like I answer this question at least once per week:
If you're interested in primary care (family, internal medicine, peds, psych, OB/GYN) you will not need USMLE scores to match at the vast majority of allopathic programs. These programs are familiar with DO's, our training, value our primary care emphasis, etc. If you intend to do a fellowship after residency, don't worry. They could care less about board scores at that point and are more interested in research, letters of rec from leaders in the field, etc. You likely will NOT need USMLE scores to match in to less competitive specialties: Neurology, PM&R, Anesthesia, etc.
However, it is DEFINITELY in your best interest to have USMLE scores if you want to match into competitive specialities or non-competitive specialties at very prestigious places (internal medicine at Mass General for example). It really has nothing to do with DO discrimination and more to do with individual program director's comfort level and knowledge of the COMLEX, what it measures, and how it is scored. You can't assume that all program directors know about the COMLEX, though many in fact do...The NBOME needs to be more proactive in educating allopathic program directors about the COMLEX, but they have not seriously done so to date.
All that being said, there is really no harm to taking the USMLE as a DO. Since it is computer based, you can sign up and take it just about any time you want. I took both steps of the USMLE after studying for the COMLEX (which you have to take according to a schedule) and did very well on both sets of exams. The exams are very similar. Studying for one is good preparation for the other. If you do well on the COMLEX you will likely do well on the USMLE. Yes, it is an extra cost for DO students, but c'mon we're talking about your career here. What's an added $400 dollars 20 years down the road?
Personally, I felt more confident interviewing at prestigious allopathic institutions having USMLE scores in hand. I just felt like there was NOTHING that the other candidates had over me in terms of grades or board scores. I was well received as a DO-student at these places too. For the record, residency programs could care less which step III you take (which you take during internship) once you match. They just want to step I and step II scores to level the playing field in the selection process.
Finally: Every year several hundred, perhaps thousands, of DO's match into allopathic programs without USMLE scores. Some of these programs are very prestigious. In the long-run I think that this is good. As program directors have more experience with these DO candidates and become familiar with the COMLEX, DO students will be less burdened by having to take two exams. Every year it gets better and better for us. It will be even better for you. I think that the day when program directors truly see the COMLEX and USMLE as equivalent licensure exams is still about 10-15 years away. Until then, I advocate that DO students take the USMLE provided that they did well on the COMLEX. Think of the COMLEX as a "screening exam" for the USMLE. If you nail the COMLEX, which I did, you will score well on the USMLE too and it might buy you a couple extra residency interviews that you might not have gotten otherwise.
Don't sweat it: Many have gone before you in this process and there are still many to come...