Is there any REAL advantage of using vStorage API for VMware backups?

I'm no VMware expert, but one advantage of the vStorage API's is that they allow 'changed block tracking'. In effect the backup software is supplied with a list of which blocks have changed since the previous backup, and it can ask VMware on which storage these blocks are stored.

This allows the software to retrieve the minimum amount of data directly from a SAN. Ultimately it means backups are very fast, LAN free and are “localised” to the backup server.

The backup software we use, Veeam Backup & Replication, performs very fast backups this way. Straight from the SAN over a single 1Gb iSCSI connection.

If you weren't using NFS, the advantages would be a lot more obvious.

FC and iSCSI storage on VMFS filesystems means you need something to grab the data out; vStorage APIs serve that purpose.