Trying to be a dutiful member of the community, I have previously alerted of accelerating cross-posting here, where I even posted a single-phrase update yesterday.
But this does not mean I disagree with the practice (or that I agree). Although Nick Stauner's answer is a tour-de-force, I have a few(...) things I would like to say on the matter:
Let a question that is on-topic on all sites that is cross-posted. Why should the practice be considered unethical or wasteful? These are the only two issues that one could raise against a cross-on-topic cross-posted question: either it wastes resources (time, attention, queue space etc) or it gives the asker an "unfair advantage". Or better, exactly because it creates an advantage by wasting resources, it should be considered unethical (me being an economist).
So, does it? Apparently yes. It appears on two different url's, it is read by different users... moreover, it may be answered in essentially the same way by two different persons, so even more waste...
But why is this a waste? Another important issue in Q&A sites is "why and who do we answer for?" In most cases, here and in other sites, the attitude is "we answer for the asker, but also for posting knowledge on the web for all future askers in need" (an attitude with which I agree). In that case, one could even argue that not cross-posting a cross-on-topic question is the attitude that should be considered unethical, since it deprives the other crowd of an interesting question that may receive a useful answer. And why is it a waste, if two different answerers provided the same answer? Answerers are doing it exclusively for their own psychological reward, so, each one separately was not hurt -on the contrary.
But, it is a waste for the community seen as a whole, because, the second answerer could have answered another question that now may have gone un-answered, making it an unfair situation for the asker that didn't get his answer...
But wait, what "community as a whole"? If the SE world was one community, why do we have many sites? The answer to that is rather obvious (practical, technical, socio-psychological reasons). But what if the SE word of websites was one integrated interface, where the existing Q&A sites where "thematic tags"? Would we then say "please don't tag each question with more than one thematic tag"? In other words, is there any essential difference between cross-posting a cross-on-topic question, and multi-taging a question?
These back-and-forth arguments can go on indefinitely. In other words, this is a classic case of a situation where either stance will to some degree violate some accepted principles and values, sometimes the same principle by a different route, a usual situation in human affairs. Cross-posting may be considered wasteful and unethical, but it can also be considered non-wasteful and ethical. Not-cross-posting may also be considered unethical...
In which case, we have to turn to "unintended consequences": what if cross-posting was accepted, even encouraged? Is there not a real probability that pretty soon, a visible number of users would start cross-posting even off-topic questions? Increasing many times the burden of each community to clean up the newly created mess, and then really causing a waste of resources?
To consider this probability as tangible, one should side with the opinion that humans may be controlled by ethical values, but they are driven by self-interest -and the Internet is notorious for lowering the power of the former (this is one of the things that I really like about CV by the way -it is so un-Internet from that perspective). I do side with this opinion, and so I consider this probability an important hurdle in embracing cross-posting. In a sense this is what Jeff Attwood advanced as a "slippery slope" argument in a more direct manner.
Cross-posting may have principled arguments in favor of it, especially as SE Q&A sites multiply and the degree of overlap increases. But my hunch is that the extend to which the practice will be mis-used, once it is openly allowed/encouraged, will outweigh the benefits from it.
Under this light, suggestions like Nick's "Don't copy-paste; Re-frame your question" are an excellent way to make productive and positive the "illegal" practice of cross-posting: "Give us a good reason to tolerate what you do, although we won't officially embrace it".