But if you don't have a microphone how could you use it like a walkie talkie to communicate with your mates as if you were spies???

On a serious note, at least this is open source, so you can inspect and understand exactly how it works rather than assuming the black box behaves as advertised, without doing something nefarious behind the scenes.

And if you're that bothered about that kind of thing being exploited against you, you should look into removing the hardware components. There are places that offer that as a service for phones (mainly removing cameras), so I'm sure some will do it for Pebbles. Assuming you don't want to DIY.