This post describe the architectural differences of SYSVOL replication on Read-Only Domain Controllers in Windows Server 2008 (download link) – FRS versus DFSR. This makes another good case for taking the effort of getting your domain functional levels to 2008 and migrating off of the File Replication Service.
While FRS can be used, it has some significant downsides in its behavior if the environment is not carefully administered. Since RODCs are designed to be placed in locations that will not have administrators or very basic role-separated administrators, this can be problematic. FRS does not contain the full plumbing to undo changes, but instead only prevents changes from leaving the DC.