Modern smartphones are always listening for "siri" or "ok google", and are regularly sending keep-alive packets, so I doubt that the battery drain for spying would be significantly noticeable, if done properly.
> why do one or the other?
Because resources are limited, even at government agencies. Effort spent hacking a device that will probably sell in the tens of thousands when they could be targeting devices that sell in the hundreds of millions just seems silly.
I agree that hacking the Echo might be pointless because it won't sell many is reasonable but that's a somewhat different argument.
Still, personally if I had concerns about privacy and secrecy I'd be looking to limit the number of devices in my own home which had an always active microphone.
> why do one or the other?
Because resources are limited, even at government agencies. Effort spent hacking a device that will probably sell in the tens of thousands when they could be targeting devices that sell in the hundreds of millions just seems silly.