I'm learning about perl, and came across this answer: https://stackoverflow.com/a/3589995/191463 the first sentence makes it sound like the first link would be very useful for someone trying to learn perl. However the question that post links to has been deleted after being closed.
Thankfully I asked a simple question about Objective-C a while ago, and got over 400 upvotes, so I have the required reputation to view deleted questions, so this is no major problem for me. However other people aren't so lucky. (In fact generally speaking the idea of reputation is that people who know more have higher reputation, compare Jon Skeet to a kid who's just starting out programming, so the people who are able to see these questions will typically need them less than those that can't see them)
The question was closed, it's not going to have any further activity and it's not going to clutter up the site. And closing is enough to discourage future questions like it (even though often they are useful)
Everyone hates finding dead links to other sites, sometimes with a bit of Googling and luck you'll find the content elsewhere, that's not the case here. Dead links are evil, Stackoverflow should not be making the internet worse.
The consideration for deleting a question should be: Is it spam? If it is delete it. If it's not does the question help make people better programmers? If it would don't delete it. If it wouldn't are you sure it's not spam...
TL;DR; Dead links are worse than a slight increase in similar questions.