I'm not sure if it answers your question fully, but taking the exam without the course is very common in our school. Our school offers a fair number of AP classes, but the competitive kids seem to feel that the only way to distinguish themselves is by taking more APs than the next kid, and they accomplish that via exam without the class. There is no way one could fit all those classes into the schedule our high school follows, with the extra period needed for labs, etc, and including the required classes, like PE. So basically these kids are trying to make their college apps more attractive, often studying for multiple exams all summer. It also seems common for kids to take exams to try and place out of requirements in college- macro and micro economics seem popular for this purpose, though I'm not sure it actually accomplishes what kids seem to think it does- though certainly depends where one matriculates. There are many kids who take Chinese as just the exam, also psychology, environmental science, computer science. For CS, our school's class is reputed to be weak on teaching, so many kids opt not to take it and just self-study, for example.
Again, I am not sure if this answers your question- I would not want my kid to take the AP bio exam without the class, for example, because I think the lab work was one of the most important aspects of the class, and it was well-done. Taking the class was also a good way to get to know the teacher well, and my DD will be using him for a recommendation- just another thing to consider, But I agree with previous posters in that I would not want my kids to take a class when they already know the material- there is enough of that in required classes as it is, and they don't tolerate it well.
