For my first 33 years of airline flying, we always appeared for classroom training or simulator sessions in civvies. Not T-shirts and flip-flops, but nice "business casual" dress.
For my FINAL 3 years of airline flying in the Middle East with QR, we always had to wear the uniform but were allowed to leave the jacket and hat at home. The instructors wore uniforms as well.
At first, I thought that this was really weird. But eventually, I came to like it. I always knew immediately, or as soon as we hit the briefing room if my sim partner was also a captain, or an F/O. It saved sometimes awkward questions if the dude appeared to be out of the normal age bracket for the seat that he occupied. We even wore seat belts and headsets. But the best thing?.... I always knew where to reach for my pen.