When I was an undergraduate, I was always a little bit puzzled by the handout I'd receive in every class warning of what a serious academic offence plagiarism was. The same handout was provided verbatim and unattributed in each class, and I used to think myself clever, arching a suspicious eyebrow at the instructor and asking, "Did you write this yourself?" seldom bringing as much laughter as I'd hoped.
But I never felt the warning about plagiarism was very helpful in making clear just why it was such a serious offense. We were just told, "Don't do it." My naive first reading of it was based on a sort of intellectual property idea, that "stealing" someone else's work and taking credit for it yourself was essentially a violation of someone else's rights, that you were cheating someone else out of they due credit.
Of course that's one element of what makes plagiarism wrong, but respect for the original author's interests isn't the only value at stake, and actually a very small one in the academic context. How does it harm me as a writer if some kid in a philosophy of religion class in Kentucky passes off my essay on Anselm's Ontological Argument as his own? Maybe if he published it widely and diluted the market for my doing the same, but in a paper that only his professor will see?
And so if we emphasize this aspect of plagiarism, we make our dire warnings of how very very bad indeed it is to steal someone else's work ring hollow, arbitrary and forced. Seriously, who cares?
The problem is complicated, because there are actually several very different values at stake, and some of them are at odds with each other. Curiously, though, these sometimes competing values actually lead to the same conclusion, albeit for very different reasons. I'm going to talk here about three here: pedagogy, evaluation, and scholarship.
Pedagogy is about teaching; the objective is to impart knowledge and understanding to the student, to help them gain mastery over the subject matter. To that end, it's an extremely useful exercise to have them attempt to explain concepts in their own words. I have been a student, and I have been a teacher, and I have never learned so much about a subject as when I tried to explain it to someone else. This is why the essay is such a common form of assignment; the cognitive effort of formulating a thesis and composing sentences that actually convey understanding of the subject matter is ferociously powerful in developing and reinforcing the student's own understanding.
Sometimes, students will misinterpret the reason for an assignment, taking the instructor's question at face value as a request for knowledge. If a friend asks you for a justification of abortion that recognizes the personhood of a fetus, it's perfectly reasonable to just hand him a copy of Judith Jarvis Thomson's A Defense of Abortion, but your instructor presumably already is familiar with the subject matter of the assignment; she's not asking you to explain it for her benefit, but for yours. Merely copying out what someone else has written has almost no pedagogic value whatsoever (beyond perhaps honing one's penmanship, typing, or cut-and-paste skills). Now, maybe you do already have a keen understanding, simply from reading the text (in which case, good for you!), but if that's the case, explaining it in your own words should be a breeze. Even if it is, the exercise is still well worth carrying out, because you will improve your understanding by trying to communicate it.
So, with respect to the value of pedagogy, plagiarism is primarily an offence against the student himself, a squandering of the opportunity to learn. And, to a lesser extent, it's a waste of the time of the instructor who reads the plagiarized assignment and tries to give you thoughtful feedback on what she thinks is your own understanding of the subject matter. Admittedly, the instructor is probably being paid to waste her time this way, but give her some credit -- she's probably not just doing it for the money, and really also wants to help you learn.
The purpose of evaluation and testing is to grade the student's performance, whether for the beneficial purpose of diagnosing where more pedagogical effort is needed, the benign purpose of certifying that a student has met some requisite standard of expertise, or the downright pernicious purpose of weeding out those deemed unworthy of further learning opportunities. (As a teacher, I absolutely detest grading, though I understand why it needs to be done and so I grit my teeth and do it ruthlessly.) But whatever the ultimate use of the evaluation, the accuracy of the results absolutely depends on not helping the student to answer the questions. The test is meant to measure how well the student understands the subject matter, without help.
Obviously, then, a student who copies someone else's work on an evaluation undermines the accuracy of the test. (Well, not always. I once had a student cheat on an assignment in a business ethics class I was teaching. Seriously! Business ethics! The result in that case, though, was actually an extremely accurate measure of how well he had absorbed the subject matter...) To the extent that the grading serves a socially useful purpose, this kind of cheating hurts everyone.
Of course, there is overlap between pedagogy and evaluation for most assignments, since we tend to put grades on everything, including those assignments which serve a primarily pedagogical purpose. We kinda have to, because students tend to skip assignments if they can get away with it without harming their grades. This complicates analysis of individual instances of plagiarism; is someone trying to get a better grade than they deserve, or just being lazy about the exercise?
An example: I once had a student submit an assignment in which he was supposed to choose a scenario and analyze it according to a particular set of legal principles. He picked an actual case, from the published decision of the court, which by itself was not a problem. However, rather than write the entire assignment in his own words, he cut and pasted the relevant portions of the court's own analysis. Now, my first instinct at this point was to nail him to the wall for plagiarism, but then I noticed something: the original case had not addressed every issue (the defense had not contested everything), so my student had taken the trouble to compose his own (quite competent) analysis of these issues. In fact, he'd been very selective in choosing only the best and most relevant sections of the text, and so it was clear to me that he had actually worked on the assignment, reading and thinking hard about it if not actually writing so much. So the pedagogical objective wasn't really being undermined so much. And my purpose in assessing how well he had absorbed the concepts wasn't totally frustrated, though it was made rather more time-consuming as I had to compare, line by line, the original judgment with his submission. If he had just identified the parts he quoted, my task would have been much easier. (As it was, I refused to grade it as written, and gave him a stern warning about the academic integrity policy and just how very dangerous to his academic career such a mistake could be. It was a teachable moment.)
This brings us to the other reason for academic integrity, which is just a matter of doing good, useful work as a scholar. The whole enterprise of research and writing is to try to make some kind of meaningful contribution to expanding human knowledge. This is inherently a collective effort, involving thousands of human minds over many generations, and this creates some epistemological hurdles. If I publish a paper claiming that the Moon is made styrofoam, it's really not of much use to anyone if it's just my unfounded assertion. I should provide sources and references, cite where and how I got my data and how I reached my conclusions, to facilitate as much as possible the work of other scholars in understanding, evaluating, and ultimately confirming or rejecting my claims. As I've posted before, nobody cares what you think. What people care about is what they ought to think. And so it's important to provide all the information you can to help them make up their mind.
To that end, we have developed various conventions about how and when to cite authorities, how to identify and attribute a quote, and so on. It's also useful to give credit where it's due for ideas you didn't come up with by yourself, not so much because the original author needs the pat on the back (though that's just courteous) but because it helps your audience to better understand where you're coming from and gives them another avenue to further investigate the ideas you're talking about.
Failure to properly attribute sources in this sense isn't so much dishonesty as it is just laziness, and the chief effect is that it limits the usefulness of the finished product. Inasmuch as we're trying to teach students the habits of good scholarship, well, of course we're going to want them to include proper citations. But mere failure to do so is really more a matter of doing shoddy work, and shouldn't be confused with the grave offence of academic dishonesty.