I guess if this is a stable id.

It can be used several ways.

If you created multiple accounts on amethyst, then those accounts could be connected to 1 user, so all different npub info decreases your anonymity set.

Also the connections of all accounts (social network) could be merged for those accounts, for all amethyst user's, then voilá a nice social graph.

I mean they only need the device id - npub, the rest is public information.