Two reasons:
1. SMB 3 (as you said)
2. Veeam - I'm still having to use Arcserve for the CIFS shares. I'd really like everything to be in Veeam as the single solution.
We are a single site, so snap-mirroring off site is not an option. We currently use Veeam backup to a Synology NAS that is replicated to a partner off-site.