Atlassian recommends disabling the XML backup both for performance and reliability. Setting up a test server and the Production Backup Strategy is better done with an SQL dump. Upgrading Confluence is better done without the XML backup.
The one operation for which an XML backup is required is database migration. For this we recommend a commercial database migration tool. Vote for CONF-12599 to add a more robust strategy for large implementation migrations. Atlassian does not support migrating to a new database.