It's very state dependant. In some states, it requires completing some sort of AOA-approved licensure examination (i.e., COMLEX, NBOME exams, etc) if you are a DO looking to get licensure. Some states don't care, as long as you pass all 3 series of test that the board has approved (usmle, comlex, flex, nbome, etc)
For example, the California Board of Osteopathic Medicine requires applicants to have passed either the COMLEX or FLEX series.
§ 1611. Credentials.
(d) Written Examination Results. The certified results of the National Board of Osteopathic Medical Examiners, Inc. (NBOME) or Federation Licensing Examination (FLEX) when applicable, or other State's written examination results shall be submitted to the board and must be sent directly from the certifier.
http://www.ombc.ca.gov/osteo_regs.htm
For Pennsylvania, the State Board of Osteopathic Medicine requires
§ 25.241. Unrestricted license by examination
(2) Received passing scores on Parts I, II and III of the National Board Examination. The applicant shall pay the required examination fee at the direction of the National Board.
§ 25.1. Definitions.
NBOMEThe National Board of Osteopathic Medical Examiners.
National Board ExaminationThe uniform written examination of the NBOME.
http://www.pacode.com/secure/data/049/chapter25/subchapGtoc.html
http://www.pacode.com/secure/data/049/chapter25/subchapAtoc.html
Virginia does not care whether, as a DO, you took all 3 USMLE or COMLEX, as long as you finish some sort of national board licensure exam
Part IV. Licensure: Examination Requirements.
18VAC85-20-140. Examinations, general.
B. A Doctor of Medicine or Osteopathic Medicine who has passed the examination of the National Board of Medical Examiners or of the National Board of Osteopathic Medical Examiners, FLEX, or the United States Medical Licensing Examination, or the examination of the Licensing Medical Council of Canada or other such examinations as prescribed in §54.1-2913.1 of the Code of Virginia may be accepted for licensure.
http://www.dhp.state.va.us/medicine/leg/Medicine2-8-06.doc
As you can see, it is state dependant. In some states, you would not be able to file for initial permenant licensure if you don't finish COMLEX. In others, it doesn't matter.
As it stands now, it makes sense to finish the COMLEX series since you're only 1 exam away from finishing the series. And the only reason to take USMLE 2 PE and USMLE 3 is for foreign licensure issues (or claim to be a diplomate of the National Board of Medical Examiners)
And the issue of state-license reciprocity is another issue that haven't been explored yet (aka whether you can get licensed in a state which requires comlex but since you have a valid license in another state, would the same requirement still apply?)