I'm not saying is a good thing, but that's a policy decision on what the NSA's mission should be. "whistleblowing" that it's happening doesn't really change anything.
You may argue that we already knew what the NSA was doing, but specifics matter. If I have a key to your house (for emergencies), you know that in theory, I could be there every minute of the day when you aren't. But you assume that I'm not doing this. The specific knowledge that I was there, and ate the last piece of chocolate cake, changes everything.
While these agencies are nominally accountable to Congress, and thus their mission is "approved", in practice, it seems they are not. Snowden's whistleblowing is a step in the direction of accountability.
Also, don't think like a hacker, think like a politician. Having the information isn't a simple boolean state. There's a difference between "the information is available" and "everybody knows, and it's top of mind". Or "people have vague worries that X and Y might happen" and "X and Y is happening and it's on a grander scale than you could have conceived".
It's one thing to "know" from movies that someone is being spied on. It's another thing entirely to know from NSA documents that you and people you know are being spied on.
Details of military battles could be kept secret during the battles and for a couple of years afterward, and sure we should keep demographic data about private people restricted to only those agencies that legitimately need that data, but those are the only exceptions that come to mind.