With the release of Windows Server 2008, the need to extend the Active Directory schema in order to use the new version of Distributed File System Replication (DFSR) depneds on few DFSR scenarios available in Windows Server 2008.
DFSR with SYSVOL: If you want to stop using the legacy File Replication Service (FRS) to keep your SYSVOL shares in sync, you definitely need to extend the schema. This is because only Win2008 DC’s can participate in SYSVOL replication using DFSR, and in order to add Win2008 DC’s, the schema must be at 2008 level (i.e. version 44). Additionally in that scenario, you must be at a Windows Server 2008 Native Domain Functional Level. You can check the schema version of your forest by looking at the objectVersion attribute on the Schema object. You can view this with ADSIEdit or with a Dsquery command:
dsquery.exe * "CN=Schema,CN=Configuration,DC=contoso,DC=com" -scope base -attr objectversion