Building cheek pieces out of mat and tape is required at sniper schools in the service. Those rifles also come with adjustable check pieces but using them is not authorized (at least not in the Marines.) You run everywhere you go through hills and water, from yardline to yardline at the range, etc, etc,... That one screw that holds the check piece will not only come loose but it most likely wont even be in the rifle when you get where you need to go to take your shout. However, those isomat cheek pieces will not only still be there but they will have your perfect sight picture. Mile after mile, mission after mission.
My guess is most people who use them are prior service. I ditched mine so others can adjust the cheek piece to use my rifle. If I were to go back overseas I'd go back to my isomat cheek piece.