It's of course total bu*****t to state that an MD is totally useless to an OMFS. And it has nothing to do with ego either. You're quite simply better of with an MD.
True, you're not going to get better at oral surgery just because you have an MD, and you'll most likely learn enough "medicine" during recidency to function completely fine, working in a hospital with other specialities.
As GR points out in his post, it's the practical aspect of an MD that matter most in the end.
I'm single degree. I work a lot with ENT at my hospital. I do orthognathic cases, iliac crest harvest, TMJ surgery, I work with plastics on reconstruction with vascularized grafts and so on, and nobody gives me a hard time.
For a long time the iliac creast was off imits for the OMFSs at our hospital, but thankfully that has changed, through the hard work of my predicessors. But when it comes to trauma it's a different story. Some of the ENTs don't like "the dentists" doing zygomas or orbits. We do zygomas and orbits, but at the risk of a confrontation with ENT afterwards. However, no ENT at my hospital can raise a a bicoronal flap, so they sometimes call us in to do it for them.
It's double standard and stupid, I know. But without an MD, we're always going to be in a vulnerable position against other specialities, escpially those that cross with OMFS.
If I were a dental student considering a career in OMFS, other than strictly private practice, I'd go with a double degree. No question.