OK, this is clearly a bad thing, but I don't think it means that your private LAN is immediately accessible to people all over the world does it? Multiple routers using the same keys means you could be tricked into logging in to someone else's router without knowing, but that would still require some way of directing your traffic to the imposter's device to begin with, such as DNS hijacking.
Knowing someone's keys would also allow you to encrypt/decrypt traffic as that device, facilitating a man-in-the-middle attack, but still, you need a way to get in the middle between two devices. This is not something that's trivial to do from one arbitrary location to another.
I'm not suggesting this isn't a serious problem, but I don't think it's as bad as, say, remote administration being enabled with a known default password.