You've hit the nail on the head. The problem with leaking, even for the public interest, is that once the material is out there, you have little control over it. The information-wants-to-be-free crowd have all sorts of half-arsed claims about how responsible they are and how they can be trusted, but they cannot be trusted not to put massive amounts of blood and treasure in danger, because they simply don't have the necessary background to make sound judgements 100% of the time.
Snowden is an self-aggrandizing douche, just like Julian Assange; furthermore he runs with people like himself, like that compulsive Judas, Greenwald, and his Brazilian rent-boy. They cannot be trusted to do the right thing. They are dangerous, naive and fallible.
Stuff gets classified for a very good reason, NOT because they are on some kind of toddler-level power trip to keep stuff secret merely to annoy tin-foil-hatters and Internet libertarians. If certain secrets are leaked, huge amounts of taxpayers' money spent to keep ahead of our adversaries is wasted; and real people could die.