Typical use case. That happens at the Open University:
1. Teachers create quiz that contributes to the course grade.
2. Students take quiz.
3. Teacher's review student's answers, and see that question 7 is faulty.
4. After students have attempted the quiz, OU policy say that questions may not be changed. The response is that the Grade for the problems question is set to 0, and the quiz regraded.
I know there are issues with ensuring the regrade is done, however the overview report in 2.0 is better than the one in 1.9, and we can further improve things later. For now, we must not break the ability to change the question weights.