I see this often in legal arguments, especially (but not only) in student work -- the writer says something like "This option would arguably violate the right to jury trial," and feels that this sufficiently distances him from the assertion that he doesn't actually need to defend it.

This sort of usage is, and should be, quite unpersuasive. If you want to argue that the option would violate the right to jury trial, argue it. But if you don't think you have enough of an argument to support the position, then don't just assert that the position is "arguably" true.

Of course, sometimes a substantial possibility of some legal outcome might be enough to counsel against risking that outcome: For instance, a prosecutor might shy away from a (relatively low-benefit) litigation tactic simply because that tactic could cause the eventual conviction to be reversed, even if that result isn't certain or even highly likely. But there too "arguably" isn't enough -- you have to argue why there is such a material risk, and why this risk is reason to reject the option.

So the word "arguably" always puts on me guard that there might be an assertion being made without any supporting argument. And most of the time, that's precisely what's going on.

My "arguably" post reminded me of a related practice — saying "Professor X says, 'This law is unconstitutional [or whatever X is saying]'" when you agree with Professor X, but (1) without expressly saying that you agree, and (2) without explaining why you agree with Professor X and not Professor Y (and there usually is a Professor Y who says the contrary).

If you want to endorse Professor X's view, be clear and candid about it; say "As Professor X says, 'This law is unconstitutional'" or perhaps just quote the assertion, "'This law is unconstitutional,'" and cite X in the footnote. That will make clear to the reader that you are embracing that assertion, rather than leaving a question in the reader's mind.

Putting things that way will also likely make it clear to you that you are now asserting something that you need to defend. And it should lead you to ask yourself, "Will the reader agree with the quoted material, and, if not, what counterarguments will the reader mentally make?" Unless Professor X is a very respected authority indeed, simply X's name won't persuade the reader. Either the quote must itself contain a persuasive and relatively complete argument, or you have to explain why the quote is correct.

This should be obvious, but I've been struck by how often legal writers (especially students) miss it.

UPDATE: Just to clarify -- I'm referring here to the use of the phrase in legal argument, where the author is expected to defend his assertions in some detail. Naturally, in lots of contexts (e.g., casual conversation), the rules are entirely different.