Yes, backup solutions for VPS providers can be helpful. Although it is not necessary to backup customer VMs, it is something that can be offered as a service or courtesy. Some people will pay more knowing their VPS is backed up to another location in case of a disaster. If you have "mission critical" DNS or cPanel Web servers on the VPS node (that is, for your business), then I would highly recommend backing those VPS up to a 3rd party provider or another backup server.
We currently backup our mission critical servers (customer web sites, DNS servers, etc) to a VPS that is geographically diverse from the main nodes. If we ever had some sort of fire or natural disaster that rendered our hardware useless, we could quickly deploy servers at another location using the backups stored on that VPS.
It all depends on the level of service you want to provide your clients