Hi all, and so sorry for the late reply,
I have been given a few very good directions for investigation, for which I would like to thank all contributions.
Concerning the question relating to "why would you", there are a number of reasons. The first is simple, because I am a student assistants to a professor who wants to quiz her students this way. Of course that is no real reason, so the second reason, how I looked at it in my own way: say you want to simulate a graded solution. As said, this type of behaviour from a quiz is a natural consequence of paper testing, but what if you want someone to have one go with the test, as well as have it timed. In that case you don't want students to change their previous answers, nor would you want to give feedback right away. A third way, in non-graded quizzes, is purely didactical : you want students to work their way through a complete exercise, but for the sake of education you want to split up a much bigger exercise (the one example I gave for instance, or a much more complex dubble pendula) into smaller exercises. You then make the students either find out they messed up as they are working on the exercise, or once they finish all smaller parts. The final goal would be to have them be able to work out the more complex example in one go (like on a final exam).
I am currently away for a few more weeks, but once I get back I will update my progress in this discussion every once in a while when I have something useful.
With kind regards,