I think the reason they're not wide-spread, is because of the fear of potential for cross-talk. You have to remember, even if cross-talk is not "audible" yet it can still be there UNDER the audible range, enough to affect your depth.
For example: I was using a blue-tooth device for my cell phone, while detecting. At a certain point, I got a phone call. And since the earpiece was under my headphone cup, I figured I could keep swinging and detecting while I was on the phone call
As I did this, I got an iffy signal, that I wanted to investigate further. But since I was on the call, I waited till I was done. I noticed an interesting thing happen: As soon as I hung up my phone, and then re-swung over the "iffy" signal, it cleaned up and came in clearer. That told me that even though my cell-phone call (which admittedly is sending out different signals than the blue-tooth-solo), was not causing audible chatter, yet it was still affecting my depth.
Example #2: One time I took a newbie explorer user out to a particular park, where I knew there was still some deep silver to be had. My intention was, to flag some deepies that I suspected where wheatie/silver type signals, so he could come over and listen to the various sounds. After we arrived, we each fired up our machines and began to hunt. Naturally, each did independant noise cancels, and then separated ourselves enough that we no longer heard any cross-talk. I tried and tried to find the "just right" signal to flag for him. But for some reason, I just couldn't find any good turf deep-silver type signals this day. And this was odd, because I was fairly certain that this park was still producing. FINALLY I got a so-so one, that I figured I'd flag for him. I waved across the park for him that I finally had one for him to sample. Here's where it got interesting: When he saw me wave for him, he instinctively turned his machine off. As he was coming over to me, I noticed that when I swung over my target now, it all of the sudden took a step up in quality. A little cleaner, and little clearer, etc... It became clear to me that even though we'd been out of audible cross-talk range, that there is still a continuing cross-talk below the audible range, that must therefore continue, until two hunters are even further apart. How far that distance is, I don't know, but it was an eye-opener to me, for sure.
Thus I would be fearful that any type of wireless headphone, even if on diff. frequencies, and even if producing no audible cross-talk, could have minute affects on depth and performance of the detector.