Reasons:
- Gets rid of the "This was closed because of trigger-happy
moderators failing to distinguish between 'similar' and
'identical'" problem by making that situation impossible.
- Improves answer discovery for users: If Google gets you
to almost, but not quite, the right question,
cross-referencing gets you the rest of the way.
- Improves answer discovery for users: Maybe the best answer
today isn't the same as the best answer 5 years ago.
- Improves gamification: Rewards are the lifeblood of
a site like Stack Overflow. A way of dealing with
duplicate questions that rewards users for
cross-referencing them is infinitely more in line with
SE's basic idea than one that punishes them with
public shaming.
- Reduces opportunity for petty bureaucrats to take root.
- Represents a more thoughtful, measured way of thinking
about software engineering. Many questions really can
have more than one correct answer, or the correct answer
can depend on subtle details. Which answer gets the
green check mark may have as much to do with the situation
of the asker than the quality of the answer.(Yes, I think SO looks like a bug tracker used as a discussion forum. They're certainly very close, including hostile project owners, WONTFIX's, etc.)
An alternative to shaming is that you could just accept this great new content that a volunteer is giving you.
Whenever a question gets a new post it gets bumped up to the top of the activity again - the same amount of possible attention as when an hour old question gets a new answer too.
What it really comes down to, though, is that when somebody asks a new question, I want to answer their new question, with all of its subtleties, in response to its particular phrasing, in its context.
Edit: In fact, I should admit that I usually only put the effort into answering old questions when it's something that has a lot of views. It's an indication that the question is linked somewhere prominent or is ranking well for some search terms. Those questions are worth putting the time in for. Most aren't.
Closing with a link to the original is cross-referencing in a way that does just the opposite: It decreases the vitality of the community by taking away opportunities for newcomers to take part in the gamification feature.
Let's frame the issue another way: What's Stack Overflow really for? Is it for people to ask questions and get answers? Or is it just a gamified Wikipedia?
SO has hurt the industry more than helped, I believe.
If a question gets asked more, maybe having more indexed landing pages for it for search engines will make it easier to find. It seems to be a popular question after all! Why limit yourself to just one answer?
Theoretically it's better to have all the answers in one place instead of scattered about, so that you can compare and contrast them. In practice it's far from ideal. Often a new question will generate a fresh take on an answer, and the diversity would be welcome.
If there's a subtle difference between the "duplicate" question and the older one, you can edit it to highlight the difference. Unfortunately that's hard to do, and you're unlikely to get the reopen votes even if you do a good job.
But why are we talking about shoulds that will never get done when a really good answer is already here? You've got users actively providing good content and your response is to reprimand them and tell them that content should be over here instead? You know, we have these things called links on the world wide web. The internet isn't running out of them.
Sometimes it feels like the mods are building a really tidy card catalog at the expense of the books in the collection.