In the military you don't wear knee and elbow pads to look tactical and professional. You wear them because you are told to. More often than not, people who told you to wear them are not around, so you don't wear them, and when they show up you get yelled at. Or they tell you to do push ups... a lot of push ups. But it's worth it; not wearing knee and elbow pads lets you climb on top of vehicles without sliding off and falling to your death, it also allows you to have proper circulation in your extremities. Plus you can stay somewhat cool and comfortable. That's probably the main reason.
Of course ArmA 3 might simulate units that are really strict about uniforms, as well as ones that don't require knee/elbow pads or just doesn't enforce the rule, so uniform customization would be a nice option, but I don't think it's absolutely necessary.