If the NSA has zero credibility and you determine that Snowden has a little credibility on apposing points, you aren't going to lean towards the person with more credibility?
It's not simply a contest between NSA and Snowden. Snowden has also said things that Google has contracted, and things that Obama has contradicted. The point isn't that any one of these entities should prevail on their "credibility"; it's that the lack of credibility that Clapper has at NSA isn't an indicator of how much credibility Snowden has. From what I've seen of Snowden's claims, he indeed does seem to be a bit delusional.
Until the NSA denies the legitimacy or accuracy of the leaked documents, there is no disagreement between it and Snowden. And if you think Google is more credible than the NSA when it comes to describing the NSA's technical capabilities, then you are the one with a credibility problem....
Rather than debate direct access why doesn't the NSA release a video of what access they do have...
While it might be reasonable to not know who the NSA is searching for, it is reasonable for the public to be informed of what kinds of information the NSA could tap, and whether the public feels that is something the NSA should be doing.
Is that a serious question? NSA is never going to tell you exactly what access they have.
Look at what Google and Microsoft are doing; that's how we'll make progress, by having the NSA finally piss off the wrong kinds of companies (read: not telcos with enormous government-granted monopolies) who will move to force the government to allow them to say what's actually happening.
The NSA doesn't like a leak - any leak. It's hard to confer credibility onto Snowden simply because the head of the NSA was angry about a leak.
The NSA isn't about to tell us, this part of what he said was true, but this part isn't. That's the hard part for the government (and for that matter Google, Facebook, et al). They can't give a full picture of the scope or scale of the story.
I can't believe we are even discussing his credibility, when you hear about the actions of our public officials.