I seem to remember something being mentioned once, that the reason for the missing hengstler on some e-11's was because they had broken off. They were simply glued to the side of the gun, and not with a bracket, as we do today, so they were quite fragile.
So I guess the reason they are required, is because they were intended to be there from the start. Kind of the same reason we are not allowed to troop in armour thats fixed with white duct tape
We try to get the intentional look and not the field repair look, done in the movie.
Hope this helps some