If you read the article, the data includes information on informants (which is probably a ton). I'd imagine there is Grand Jury information in there as well.

Plus, there is probably personal information (payroll). Not that it matters how much anyone makes (that's probably public record already), but ACH banking details and things like that.

Some of these things we can't really reduce, need to be kept online for significant time frames, and can't be made public.

That being said, I agree that we should plan on having less perfect security (and then design for that sort of world).