I don't see a fundamental difference between clicking duplicate before altering values in the form, and clicking 'Save as new question' after altering values in the forum.
'Save as new question' did not replace the question in quiz 2 with the edited question. (unless I am very much confused.)
OK I thought in the old interface that if you edited the question in the quiz and saved as a new question, not directly in the question bank, the question in the quiz was swapped out.
That would be functionality/increased usability that some teachers would like.
A teacher that I work with is finding it easier to create the questions in the Quiz context rather than in the course context. When you choose which category to display for the question bank, you'll see that there's one category at the Quiz level. If you duplicate a Quiz with questions at the Quiz level, the new Quiz will have independent identical questions at its own Quiz level. The category will be labelled according to the original Quiz, which can be confusing (and is a known bug, as Tim has pointed out elsewhere), but the main thing is that you can then edit the new questions without inadvertently altering the originals.