There is legitimacy if you assume the leaker is the only source of the information. In such cases it logically follows that trust in the contents of the leak is reliant on trust in the leaker, until other ways to authenticate the contents of the leak become available.

Once separately verified the discrediting game is utter nonsense though.

Character matters tremendously for quality or impact of information they leak. It affects the degree to which you might suspect that there is even more information being hidden, who stands to benefit or suffer from leaks, and who the power players are. This calculus doesn't change whether you're a cyber-libertarian, damage control for the govt, foreign govts, whatever. You want to ascertain the degree to which this is going to continue to be a problem for different parties.

Furthermore to assume that studying someone's character as part of investigation (both public and NSA-internal) is just mere propaganda and media smoke and mirrors seems to dismiss more mundane notions as a) curiosity about the leaker and the story, b) routine counter-intel procedure, and c) journalists having stuff to write about.

In short, you are giving far too much credit to the idea that the public is stupid and easily distracted. Come on, we all know the score here.

Propaganda tricks work, though. It follows that in order to be an effective leaker, one must be more or less impervious to muck-raking. Which is neither fair nor sane, but strikes me as plausible.

That's the ad hominem fallacy - shoot the messenger, discredit the message by discrediting the messenger even though the two have no logical relation. It's a testament to our educational system that so many people fall for it. Though part of it, I'm sure, is that it simply gives many people a rationale for believing what they already want to believe.

The problem, though, is exactly what fidotron describes: without independent validation, we must trust the credibility of the source of a leak, for that leaked information to be considered valid.

If Snowden were a character if ill-repute or questionable morality (outside the question of the morality of leaking intelligence secrets to foreign countries), it requires that you question whether any of the leaked information may have been tampered with.

Once the relevant leaked-upon agency admits to the validity of information, then definitely the messenger's credibility becomes decoupled from the truthiness of the message.

Even if the information can be verified, there can still be questions of things like lies by omission, so knowing if the leaker has an undisclosed agenda can still be valuable.

Shooting the messenger is standard operating procedure by the PR/Propaganda wing of the powerful. There were attempts to malign snowden, I think some one went on Dianne Rhem Show on NPR dissing snowden for not have passed out of High school or for not getting a college degree. That I found was the lowest of the low hit at him. He is not a hero - but he sure is not a traitor - he decided to do what he did with good intentions and in alignment of his conscious.

I wish I could upvote this 1000 times.