I have found that in order to use Moodle's question bank efficiently, I always start my Moodle site by setting up course categories first. Then I make sure to put each new course into the appropriate course category. If you start this way, the question bank, and its advantages, work very (extremely) well. If you don't start this way, it becomes a bit of an effort to move quiz questions around in the question bank, and to share them between courses. Well, this might not be technically correct, but it works well for me.
By the way, once you understand Moodle question bank, you will find that it is much better than competing products, like Blackboard. There is really no comparison (in my opinion) between the quiz capability in Moodle versus Blackboard.