I don't have any insight into the use of those scenes, but I wanted to mention how well done I feel the instance in "Make Me A Perfect Murder" is.
There's a sort of understated intensity in that scene that makes what Mr. Flanagan is relating to Kay seem all the more grave. I particularly like the part where Kay is cut off by Flanagan when she is saying that she thought she did the right thing under the circumstances - "You created the circumstances." Or when Kay says, "Do you expect hysterics" (or something along those lines), and Flanagan responds coolly, "I hope not." The mix of dignity and antagonism between Kay and Flanagan in that scene makes it one of my favorites not only in that episode, but in the entire seventh season.