That would be an accurate description of the origin of the guard, but there are a number of more offensive guards and approaches to using the guard that have been developed. See the video clip in the Ryan Hall MMA debut thread for an example.The guard is a position used to limit (and control) the opponent's ability to continue to attack. It's defensive in nature, kinda by definition (that's why it has such a defensive name… the guard…).
A more precise distinction would probably be to say that the guard (of whatever variety) is designed to control the opponent's body for defensive and/or offensive purposes.