There are a lot of spammers either automatically submitting links or manually continuing to do so long after being identified and they're not always obvious. The other week I found a network of ~20 accounts that had each been submitting one generic tech link a day for up to 1.5 years, if you browse when that network does their daily submissions you'll see almost a full page of dead links that look legitimate.

The ability to upvote dead comments out of oblivion would be useful for commenters.

Edit: as luck would have it that group is submitting a batch right now - http://i.imgur.com/MVvfoaC.png - all those dead accounts (+ aynlaplant, flag to help kill it) are spam though they don't look like it at first glance.