please find the issues below if you relay on only snapmirror transfers between sites
1. If data got correpted on source and you are not able to notice it with in the time frame of snapmirror transfer, then snapmirror will replicate the correpted data to the destination.
2. You can not maintain long retentions by using snapmirror like backup tools and snapvault.
3. Why do you need to waste costly WAN technology only for backups?
If you don't want to use other backup tools then you can go ahead with snapvault technology and also you can attach tapes to netapp and backup the data.