Common practice in many places, strangely enough its often the hot and sweaty climates. I mentioned this in one job but was basically told to do one. After someone had been using a jacket for 5-6 hours offshore in the far east, no air con, and sometimes 40+ degrees on the ground, it soon gets wet with sweat. The next crew come in and wear this over a thin flight suit or shirt. They also often stink to high heaven from the sweaty bacteria.
You can feel the wet collar and smell it. Not pleasant so I understand the OPs point.