Just because it's not posted publicly doesn't mean it won't be used by whatever parties have it. Given that this seems to be an American posting (comments about pizza and beer) and not the badly-typed Russian hacker earlier, it would appear that two parties at least have a copy of this information now (or alternatively got the information independently). You'd be a fool to assume this person is acting in good faith, they've exposed a hell of a lot of private data by posting this alone.

But, if that hacker can get in, more nefarious ones can as well. You should assume that any information stored with Gox is now no longer private.

True. They have not included it, they probably have it. Maybe there are other people who already have accessed it. He is effectively considering it in hands he may not want.

That's because they may be selling the rest of it. A 'nanashi' has already posted on Bitcointalk their terms (100btc for the entire dump, 10btc for 2gb etc).