I would hazard to say that this model is most common in the academic world.
Like many programs, we have an "emergency surgery" service which includes some, but not all, of the trauma/critical care attendings. That means that there are attendings who purely do emergency surgery, no trauma, no critical care.
One in particular is on service every other week. However, she also has an NIH RO1 so she is really quite busy outside of clinical responsibilities, not to mention being in the call schedule for ES q4 when not on service.
Similarly, our other trauma/CC/ES generally have a schedule like this:
week 1: on trauma service through weekend (no call)
week 2: on ES (likely in the call schedule a few days/weekends)
week 3: rotating in SICU (in the call schedule)
week 4: "off" aka academic week, but again, still are in the call schedule, have to teach conferences, go to grand rounds, do research.
As has been discussed recently, this service has little clinic - in fact, only one day a week - which is shared by all 4 attendings. No show rates are quite high. All attendings are essentially expected to go to clinic every week.
Plus, as has been noted, despite a "lighter" work schedule on paper, their call nights are VERY busy and unpredictable, and the patient population is hardly conducive to a long, fulfilling patient-physician relationship. Trauma patients aren't the ones bringing bottles of wine and thank you cards to their follow up clinic visits (or for that matter, when they get their second ED thoracotomy a year later for another GSW). So this is why burnout is relatively higher for these specialists (like ED).