After 9/11, they use to prefer us to wear them, as an extra preventative for security. It may deter a would be baddy by seeing us out among the crowd. During early Iraq/Afgani wars, they specifically made us wear them for this reason. As yrs passed, as I went back a few times, they had us fly in civilians later on. It’s usually dictated by our unit/upper echelon SOP, which changes quite often, depending on circumstances. However, if you’re a soldier that isn’t flying for military purposes, then chances are you shouldn’t be wearing a uniform. If you see soldiers or marines flying in uniform, 99% of the time, they’re probably on duty.