When I see a post with problems, and there are comments1 raising the issue that are more than a few hours (or few days, depending on the site) old, I tend to check the poster's "last seen" date to see if he's been back to the site since those comments were posted. If it's likely that comments have been seen but the post hasn't been fixed, I am more inclined to take a stronger action (downvote, flag, vote to delete) than I would if I thought the user hadn't yet seen the feedback and had a chance to address it. As a moderator I also use the "last seen" information to decide whether to delete comments (if they're ones he should see but that shouldn't stick around forever).
A recent "last seen" doesn't guarantee that he's actually seen something, but it's evidence. I suppose I could look at his recent activity to see what he's actually been doing, but that's more work. (I'm aware that a stale "last seen" doesn't mean he hasn't seen the comments, but that's not the issue here.)
By the way, since all your activity is public, protecting "last seen" only protects your reading/voting activity (if it's the most recent thing you've done). If you're active on a site, you're already leaving a trail of timestamps.
1 Or, more rarely, an "insufficient explanation" post notice.