You can use them for voice commands, or to take pictures of whatever you happen to look at. That makes them very limited even just as cameras - a fairly large proportion of the pictures I take involve moving the camera (my phone) into angles I wouldn't get my head into with comfort.

If cheap enough they could be fun as "I want a picture too fast to get my phone out and happen to have my sunglasses on" cameras, but that's not exactly something you'd expect to get you huge use. If they were cheap enough and they didn't upload everything to Facebook (which they apparently do), it might be something you'd think is a fun "nice to have" you might not use very often, but hardly something many people will go out of their way to buy at a large added cost.

When it then also costs 3x a regular pair of Raybans (UK prices at least) and if you're happy to look like you're going around spying on people you can buy a pair of cheap spy glasses on Amazon for 1/10th the price, I don't understand what market segment they think they're going for.

As just a camera, if they were detachable and visible enough that you're not looking like you're trying to get away with something, I'd be far more interested. If they're going to be permanently mounted, they need to be invisible.

Something like that would be the beach head into the AR future.