When you purchase MSB, you are licensed the existing version. Updates are provided as a courtesy: MSB is provided ‘AS-IS’ but, at my sole discretion, my policy is that keys allow you to install and use any maintenance updates released within one year after your purchase, if released (For example, I cannot be held responsible if Apple makes some change that makes a fix impossible.)
Update: Beginning in version 6, MSB has adopted the familiar ‘major version’ model. Your key will unlock any minor revisions. A grace period is provided if a new major version is released shortly after your purchase.