You know what I would want my phone to be doing? To be actually always listening. So that I could just say "Ok Google" at random and interact with Google Now without having to unlock the phone or have it constantly hooked up to a charger. That very simple thing would probably quadruple the usefulness of the whole thing. But Google probably has some "smart" reason for never ever allowing that. sigh
(I mean, what the hell - deep in the options of Google Now I can actually allow the phone to be unlocked by my voice - but if and only if it is plugged to a charger. They had to explicitly code in a special condition to make Google Now less useful.)
(and yes, I know you meant other kind of "always listening")
FYI, some phones actually have this (the Nexus 6 for example). IIRC, not all phones are always listening because of battery concerns, but some phones have chips specifically for listening to "hot words"
(I mean, what the hell - deep in the options of Google Now I can actually allow the phone to be unlocked by my voice - but if and only if it is plugged to a charger. They had to explicitly code in a special condition to make Google Now less useful.)
(and yes, I know you meant other kind of "always listening")