You might choose to perform your backups from the global zone in the following cases:
You want to back up the configurations of your non-global zones as well as the application data.
Your primary concern is the ability to recover from a disaster. If you need to restore everything or almost everything on your system, including the root file systems of your zones and their configuration data as well as the data in your global zone, backups should take place in the global zone.
You have commercial network backup software.