the trips would be long-distance, most likely overseas, out-of-the-country, and including midair-refueling in the future, most mission might be in totally unfamiliar geographical areas, as well as there would be more than 200-250 soldiers' lives on board and likely over battlefield, you would like 4 crews than 3, even with highly automated avionics onboard.  Long-time flight Fatigue is an important issue, even with highly automated systems.  Too much and too concentrated workload in potentially hostile, high-tension battlefield environments is a serious issue.  Civil aviations do not have such kind of problems.