I'm going to take this opportunity to actually list reasons!
4. The "DO" degree doesn't get you the name recognition that the "MD" degree does
5. DO's have to take both the COMLEX and USMLE to apply to ACGME residencies, which they want to do because...
6. AOA residency options are limited in certain fields, and there are concerns about the quality of AOA programs in general
7. All top academic residency programs are associated with MD-granting institutions
8. There is concern that some of these academic residency programs discriminate against DO applicants
9. Some OMM/OMT is complete pseudoscience and it's a bit annoying that the "extra class" in DO education covers that
10. There is a concern about real/perceived bias against DO's by other physicians/the public/whoever
11. All prestigious/well-respected medical schools grant MD degrees
12. There are for-profit US DO schools, while there are no for-profit US MD schools; accordingly there are concerns about the quality of education at places like these
13. DO schools typically have lower/much lower matriculant stat averages than MD's, which in itself can lower perceived value of the education, but also...
14. ...makes it seem like DO students attended because they weren't smart/talented/hard-working enough to get into MD schools (which isn't necessarily the case, but often is)
15. The osteopathic philosophy A.T. Still espoused is defunct, and with it any practical differences between the goals/methods of osteopathic vs. "allopathic" medical education, which means...
16. Eventually the programs (AOA and ACGME) will be fused and the degrees reduced almost certainly to a universal "MD" degree (see #4)
17. Some DO programs require a non-refundable deposit prior to matriculation, while I've never heard of this with MD schools
18. DO schools are often more expensive in general
19. In addition to what's covered in #5-8, the majority of DO students enter primary care fields, and besides that fact...
20. Simply coming from a DO school statistically puts you at a significant disadvantage in matching to competitive residency fields.
That enough?
Edit 6/12/15: Really late edit but I don't want to bump the thread.
@TreadLightly below hasn't posted in almost a year as of this edit, but to address his/her concern about point #16, I'm referring to long-term inevitability. The US medical community has recognized that two GME systems is counterproductive and unneccesary, and eventually the same will be seen for having two medical education paths.