This should help those of you who, like me, couldn't understand why cross-posting is discouraged, when one's intentions are good (an interest in getting the expert opinion of two diverse communities regarding a question that is on topic for both).
I've discussed the no-cross-post debates over the months and I never could understand what was wrong with posting an identical question on two sites when it was perfect for both sites as-is. A question that I thought was fair to cross-post was:
- Valuable to both communities
- Worded perfectly for both communities without any change or "tailoring" needed
- with the intent of getting two diverse kinds of answers on the question
- not just to get an answer faster
This belief was confirmed by answers like this:
In the edge case where the question is appropriate on more than one
site, leave it on both sites and let the users of each community
benefit from the information.
Here's the real underlying problem: The SE Network was not designed for duplicate questions across sites. That's the issue to which there's really only one simple solution: don't post identical questions.
Because the SE Network is designed to work as a unified tool, combining questions from all sites, identical posts are oddballs.
With the way the Stack Exchange network features questions from different sites, if we didn't discourage cross posting (even when the question is very much on topic in both places), we would see duplicate questions featured on the front page, which would look a bit weird.
The whole "It's ok if you tailor your question to fit each site" idea, which is now quoted/referenced in all related meta discussions, originally came from Jeff Atwood himself (see his answer), and that solves the duplicate issue. Other than that, there's really no issue with duplicate questions, as long as they're good questions totally on topic for both sites.
Some questions can be perfectly on topic for two sites with identical wording, but the network was simply not designed to handle duplicate posts. Change the wording and tailor the title, even if it's already good for both sites - just make it different (This argument assumes that the question is truly good for both sites) and it's ok.
To anyone who says duplicate questions a bad in all cases, that's simply wrong! When questions are cross-posted with the right reasons in mind, in the right situations, there's nothing wrong with them, except the issue that I mentioned (the SE network's unified functionality).
So when you discourage users from cross-posting when the question fits well on two sites, explain this to them. It's not that the "tailoring to each site" is always necessary, it's just that duplicates don't work well in a network where questions from all sites are featured on the same page.