There are two ways of seeing what adding a question in a quiz means. The first one is the one proposed by the original data structures: questions are created in the question bank and then added to a quiz. The second one is the new model: you add a question directly into a quiz, and as a convenient service from the system, the question also gets added to the question bank for later use. This became as clear as it is now to me only when talking with Ray Lawrence in MMUK09.
All the implications of either of the ways to understand it should be supported.
When adding a question using the 'Add question' button, there should be explicit feedback to the user, informing them about this: "Your question was added to the quiz. In addition, a copy of it has been placed in the question bank, in the category X, to facilitate later use". This is the only way for users to learn this happens (except by accident).
I always forget, if this is also reflected in how editing questions in the question bank or in the quiz works - if you edit one, does it change the other?
Wondering about the paradigmatic change that the new Quiz editing UI introduced (still for my thesis), I found that I've been not practicing what I preach in the above. Alas, this issue.