Random numbers were introduced in version 4. So starting with Flash 4, very effective quizzes could be programmed. I can usually create programs backward compatible to this version.
Flash movie compression was introduced in version 6. This is a very useful feature.
All new applicatons that I create are still backward compaitble to Flash 6, if at all possible. (Usually it's possible, though it takes a little more developing time.)
Sometimes I just need to use something that only works in a newer version, so I do this as a last resort.
It's my feeling that a good programmer always creates things as backward compatible as possible. This way you serve the largest possible audience.
Is it really important to support versions of the plugin that are 4 or 5 versions older than the current version? Well, if I were one of the ones who could not use your creation because I was on an older computer that didn't support the flash plugin needed to view your creation, I would be depressed.