You need to sync it anyway. Having that phone with you all day also means exposing it to a lot of risk involving theft, drops and other kind of damage. You need that sync for backup purposes.
I agree actually having it on the phone is great though. I use DeX a LOT, it's a great way of working when I don't have my laptop with me but do have a docking station available (e.g. at the office when I forget my laptop or just dropped in unplanned)
Backup is a simple one way sync, but like you said, it is needed. It could still be private, if backup to another of your devices is made when your phone connects to your home WiFi.
Of course, that scenario would only become the norm, if there's mainstream demand for that. By and large, there ain't.