That is entirely irrelevant to people who are still getting hacked because of these hoarded vulnerabilities.
I don't follow. Could you make this argument more specific? Who's doing this attacking? What kinds of vulnerabilities?
Stated as vaguely as you have, that's a hard argument to rebut. I tend to agree: the world would be a safer place if everything was publicly disclosed immediately. But even NSA's opponents tend not to support that as a policy objective.
"Enemy" is a weird word ("adversary" is probably better), but however you slice it: NSA's mission includes real ones.