I personally don't have anything against it if it's done well but the official Stack Exchange policy on intra-StackExchange cross-posting is more clear on the topic:
Allowing cross-posting is a slippery slope.
If you might have slightly better odds of getting an answer by posting it on two sites, well, by gum, why not maximize your odds by posting it on twenty sites!
There are some questions which fall into grey areas between sites, and I think it's OK to ask and delete, then re-ask if you feel you have asked on the wrong site.
But as a general rule, do not cross-post questions, please. Pick a site and go with it.
##I don't completely agree with this policy.
I don't want to confuse people too much, but the Internet is bigger than StackExchange sites and most things (be that programming or Ubuntu) have more than one support channel. I essentially don't think it would be fair for me to moderate in a way I don't behave.
A common cross-posting behaviour I engage in is asking for help with web development. I'm a StackOverflow member so [more frequently than I'd like], I'll ask a question on there and immediately ask on IRC (linking to the question on SO). I do this because this is my $DAYJOB. If I'm stuck behind some blocker, I'm losing money to it and I want help ASAP. It's completely selfish, but equally true.
With Ubuntu, I used to be a fairly active UbuntuForums poster. When I started here, I would very often post something here or there and then cross-link, or again, use IRC. I still do if it's urgent but thankfully I haven't had many super-serious issues recently.
I think I'd be a massive hypocrite if I slammed every cross-posted question as some sort of Simpsons-did-it-first-duplicate. What makes two diverse StackExchange sites more "special" than disconnected support avenues like Ask Ubuntu is to Ubuntu Forums or IRC? The same mechanics apply: you ask for help; people invest time to help you; you win.
But that doesn't mean I support all sorts of cross-posting; it's incredibly important that questions are maintained. By starting a question anywhere, you're creating a social contract: you're asking people for help and if they see an answered question, they might put [considerable] time into giving you a good answer. If you get an answer somewhere else and don't update your question here, you're wasting our users' time and I'm not okay with that.
My view on basic guidelines would be:
##Find the best location for your question.
There are a lot of StackExchange sites, some more active than others. Remember that activity is a double-edged sword. For example, questions on the front-page of StackOverflow are on the second page within a few minutes. On a slower, site, even if there are fewer users, more people might see your question.
##If you do cross-post, cross-link them!
Child posts should have links to the original post and if you can, try to keep the parent posts updated with a list of places you've cross-posted it to. This means that users of any site can quickly see if there have been any answers. Or post their answer on the main thread.
If you're honest and up-front about it, we [moderators] are going to get fewer flags about it. People can track things and see if your other thread has any answers.
##Update all your threads as soon as one gets a result.
This is probably the most important point. They you're questions so it's your responsibility to tell users on all the sites when you no longer need help. Doing this means they're not going to waste time fixing something that is already fixed.
Do this by either answering your own question with a link to the other answer, or in the case of IRC, say what's happened, giving the user credit. Chances are you won't be able to accept your own question if you're answering it yourself, so just flag it up and we can close it for you.
Yes, this is more work for you but that's the cost of responsible cross-posting.
This is slightly different to the path other SE sites tread. If you cross-post something on another site with a different policy, they may well outright delete your thread. That's their choice.
As a deterrent against poor cross-posting, if we notice you cross-posting without taking these steps seriously, we'll give you a warning. Twice and we'll take action to protect our users from you wasting their time.