This is a very interesting concept.  One of the drawbacks to pure fiber is the need to maintain power at the end users location in the event of a power failure. We have small battery backups that keep the phone going to a short period of time but eventually they quite as well leaving us without a working landline.  Not a big deal to some (including me) but from a safety perspective it can be a big deal.  Having a copper pair in along with the fiber gives the provider the ability to power the ONT at the end users location, keeping the service active non-stop.