Couple of reasons for the question really.
I come from a target shooting background so have used diopter open sights quite a lot! For fun I sometimes do our HFT course with open sights. However it looks to me like the rules forbid them.
However, if they are banned, what about those rifles that come with open sights and people **** a scope on and leave them there?
And of course your point of using open sights for the close ones! A blurred 10 yarder is Ok if you are an early shot, but later on it's very difficult to find a a blurred silver paddle behind a blurred silver plate!