Interesting. I am an MD, fellowship trained in sleep (along with pulmonary and critical care). After I finished fellowship, my program director asked if I wanted to sit for the BSM exam as well (where I trained was an accredited BSM program as well as AASM, which may be the difference in where watto trained vs where I trained).
I chose not to sit for the exam, only because I did not want to be pigeon-holed in to seeing primarily insomnia patients (which would happen if you were BSM certified). The other sleep MDs would get the OSA (read: PSG generating patients), you'd be stuck with the insomniacs (most of whom don't require a PSG) and as a result your revenue generated would be much lower (and don't let anyone fool you, the revenue you generate is watched, and directly tied to your future compensation).
Knowing I have the knowledge to appropriately treat a patient with insomnia and that I know the nuances of how to treat said patients is a valuable thing to have in your back pocket, but once you hang up that BSM certificate, be prepared to see a paradigm shift in your patient population. Once the word gets out that there is a BSM certified insomnia treating MD, it will dominate your practice (if you don't control the number you're willing to see).
I'd let the PhD's win this 'battle'. See a few here and there to keep your skill set up to date...