I'd vote for an explanation on the profile page, about the visibility of this and other fields. Then those who feel spied on, can either choose to use a more anonymous display name, or choose to not log in when just browsing the site.
I often check the "seen" field to see if someone might, or might not, already have seen comments on posts. Or is likely to respond to new comments.
Like: if I have postponed a downvote until the poster has had the opportunity to see and act on comments. Or if I asked for more information and am trying to determine if the question asker is likely to respond. And this does not only apply to recent posts: being lazy, I sometimes also peek into it when considering answering really old questions. So: I actually use that field, and can't think of alternatives that would work for me too.