Nice for situations when one's phone is lost. OTOH when owners would like to hide their location, they would have to use Faraday cage to cut off all data communication. Again a situation of a little more security for a little less privacy.
I think the inability to control when something is on or off... *especially* when that corresponds to an inability to control network connectivity and traffic... would rank among the most serious security/privacy issues. Although it isn't clear it would be implemented in such an absolute fashion, doing so would eliminate the most effective defense a cell phone owner has against a range of threats. So many parties will push for that approach. I think those who walk around with always on cell phones would have difficulty appreciating how serious such a loss of control really is. However, this is not an issue that is inherently limited to cellphone contexts. Conceptually, it could come to affect many forms of technology we are used to plus unknown new technologies in the future. Would you want all your computing devices locked into always-on/always-connected mode? How about SmartTV's and other home electronics? All the IoT devices that are seeping into lives? Also consider that many such devices may have embedded cellular radios in the future, along with non-removable batteries or caps or some other power source that isn't easy/convenient to disconnect.
The problem is that using a Faraday cage with a phone that's trying to connect will drain the battery.
Yes I would use Airplane mode + Faraday cage, hoping that this new feature would not drain it too much.
According to Snowden stuff like this is already possible with modern phones... Wake on lan kind of features... Remote wake mic etc.