Let's say we are in a room full of iPhones.
If "Hey Siri" could be enabled by default without a constraint like having the phone plugged in to power, when one of us said, "Hey Siri," all of the iOS devices in the room would start listening and responding at the same time.
It may be true that the "Hey Siri" feature in iOS 8 is primarily configured the way it is in order to not kill the user's battery. But, that configuration works well in several use cases:
- In the living room of an iPhone user's home..
- In the car where there is only one (or two) charging points.
- In someone's office.