You are right that it is a sentence you would rarely see. Titles seem only to be used on airline tickets these days. But you do hear them used informally. A man might to refer to his wife as "fruen" when talking to his mates, much as "the missuss" is used in English. Similarly, "frøken" is sometimes used to refer to a girl or young woman.
I guess that it is useful to know these words even if they are not much in use, if nothing else to know what to pick when ordering airline tickets and we need some sentences to teach them.
BTW: My little brother was convinced that the given name of his first teach was "Fru" when he started going to school.
I'm only concerned that people would use this literal translation, as opposed to a more meaningful translation from the English sentence.
While you could probably find some instances of both fru(e) and frøken, they wouldn't be used in the same context as the English sentence. So while learning them might be useful, I think the sentences teaching them should be more meaningful. The use should perhaps be explained in the "Tips and notes" section?
Reading the English sentence I assumed that the context was that the speaker was filling out a form (e.g., when buying plane tickets as mentioned above) and needed to know whether to write Miss or Mrs. I would have been somewhat puzzled if the Norwegian translation had been Er hun gift? :) It seemed like a perfectly meaningful example sentence to me.
It is used very frequently in the South (the Southern states in the US). They still hold very dear to what they refer to as "southern hospitality." They also use "Sir" when referring to men. It's not just old people, or rednecks - it is considered deeply offensive to not use these terms when dealing with elders, strangers, customers, and people in positions of authority (teachers, police, employers). As a transplant from the northwest, I had to learn real quick so as to not offend; but now that I've moved back to my home state, I still tend to use those terms, and I get the evil eye from everyone here who hates to be referred to that way, even though I'm just showing respect.
Kids call their teachers by their first names. Famous persons(celebrities, CEOs, athletes) would perhaps be called by their last or whole name when written about in the newspapers, but this is probably because it's easier to recognize the persons, because many share the same first name. The PM is often called by his/her first name, however, so there are exceptions to this rule.