As regards SIN, I've seen the Concorde section quite busy and as LTN Phobia points out, it is entirely possible that the whole area might be needed by F passengers. I have seen F passengers being told they would have to wait for the (much lamented) F dining at PHL because no tables were available, so I think it's perfectly reasonable to suppose that some of these locations couldn't accommodate F passengers if any significant number of CCR holders and their guests were let in.