That's a good idea. I can subscribe; however, it seems like it would be beneficial to HN to keep the links clickable. I still don't really understand the need to kill them though.
No 'link juice' or anything would ever flow to those sites.
And I doubt the folks that look at the site using 'showdead' need protection from malware or spam sites.
The one reason I can think of is that any clicks from HN would lead to those users re-posting under different accounts because they got some traffic. And in that way it might be beneficial to really get rid of them.