Have had a lot of "Do I take the USMLE" talk at my medical school over the last two weeks as board exams get closer. A good amount of my classmates have backed out of the USMLE simply because they aren't scoring well on the NBMEs (barely passing, around 200). That being said, I think it's very valid to back out if you are scoring 1 S.D. below the mean on the NBMEs within a week or two before your scheduled exam. Getting a 200 on USMLE won't help your options anywhere.
I definitely think scoring > 215 on the USMLE will help your application, regardless of what you want to go into. Remember, the average score is a 229, not a 240 as SDN makes it seem like sometimes due to all of the high scores being posted. I'd figure with a 215 or 220 USMLE you would be competitive for at least some Primary Care MD Residency Programs. You wouldn't be one standard deviation below the mean (< 210) either. Not to mention that there are a lot more factors that go into Ranking placement besides your step 1 USMLE Score.
That being said, multiple graduating 4th years that I know matched into good EM Residencies while only taking COMLEX. They highly regretted not taking USMLE though, because at the end of the day, it limits your options on where you can apply and get interviews at. Not only that, but Residency programs are beginning to merge and by the time its 2019, a lot of these programs will have merged to where they take USMLE or require/favor it over COMLEX.
And as an aside, it's very very tough to score above a 230 on USMLE (contrary to how easy it may appear due to the high frequency of 240+ scores on SDN). And it's even harder coming from a D.O. school simply because we aren't taught to the USMLE board exams. In our curriculum, we had barely any Biochemistry during our second year. I started prepping Biochem and doing USMLE-Rx back in January because of this. It took a lot of grinding and a very hard work ethic to even top 220+ on these NBMEs. So, don't get down on yourself and depressed if you aren't scoring as high as you thought you would on the NBMEs. It's a completely different test than COMLEX that requires a lot more preparation and work outside of class material.
TLDR: Take USMLE if you can score within one S.D. on an NBME (probably > 210 on NBME). If you can't score 210 or above on an NBME, don't take USMLE, and realize that this will limit your application, but it's definitely not the end of the world if your goal is a primary care residency.