Shedding a bit more light on the ethics of the action of leaking this data, the reason why a lot of people feel conflicted about the right- or wrongness of the action, is because it is perfectly acceptable from a Virtue Ethics point of view: The fact that leaking private data would be wrong in the vast majority of cases is inconsequential because the data being that of a (generally agreed upon to be) evil organisation ascertains the moral character of the actor (in some sense).
A strict moral code like the C.I. is pretty dangerous when it forces you to disclose you're hiding Jews in your attic (sorry for the "Godwin", but that's the counterexample I was taught). It's even worse if done half-assed "well obviously you don't need to take it that strict if it leads to bad things" because then you can argue your way around everything--without even having to consider "does this make me a good person?".
And from that point of view, for me, the real question to ask: Would this person (or persons) have done the same thing if the emails did not belong to Scientology or neo-nazis or similar?
However I might change my tone if something really serious is uncovered here. I think whistle blowing is a good thing, and this seems like the next best equivalent. Of course, that is a ridiculous position to take, essentially giving any random hacker the license to read through random personal email and blow the whistle on demand.[0] So I guess that position isn't tenable, either. But I know it'd be hard to be critical of a hypothetical hack that revealed or delivered proof about illegal activities by certain groups, be it Scientology, large-scale financial stuff or neo-nazis.
A more responsible release would probably only include incriminating or otherwise extraordinary emails, preserving (sort of) the privacy of most people. But arguably that's so difficult and personally dangerous in case of large data hacks or leaks that it's equivalent of saying a responsible release isn't viable.
[0] As determined by them, but that problem applies to all kinds of whistle blowing.