We are much better at faces than we are at voices:

• Imagine I show you 10 photos of faces. Then I come back 15 minutes later and ask you to pick them out from a set of 20 faces.

• Imagine I play you 10 audio snippets of someone talking. Then I come back 15 minutes later and ask you to pick them out from a set of 20 audio snippets of people talking.

That said, I agree with you that primarily this technology would be useful for surveillance.

If you can zoom in on any given conversation happening at New York Penn Station, you're basically in the Bourne Identity, with all the privacy-destroying overreach that entails.