I was having the same thought as taylorbox regarding using VMFS for storing backup.
1. Installing an ESXi to retrieve backup data thru VMFS is much faster than installing Windows to retrieve backup data thru NTFS.
2. If all ESXi hosts are down, you will still need to setup a new ESXi host anyway in order to run the backup VMs.
3. Chances of VMFS corruption - never experienced before. Chances of NTFS corruption? Quite often. At least an virus attack to delete files on an VMFS volume is highly unlikely.
4. Simulaneous connection (write) in a SAN volume is supported by VMFS but not NTFS which will lead to corruption. (Having 2 backup app writing to the same volume at the same time etc?)
Having said the above, someone reminded me about the big difference between using the storage for backup and replication. For replication, it will be VMFS instead of NTFS.
However, since backup will be compressed and dedup, we will need Veeam to restore it, or use extract utility provided. Thus, NTFS will provide direct access to restore the backup from any connected Windows computer or even execute the backup VMs to boot up immediately thru vPower. With VMFS, we will add an additional step of mapping the VMDK files into a Windows VM etc to access it. That's the only disadvantages I can agreed upon, apart from the 2TG limits.