The data that the NSA is valuable precisely because it isn't anonymous. You can't make any worthwhile statistical discoveries with truly anonymous data because you don't really know what you are measuring. Any information that they don't strip before releasing it (which they won't) could be traced back to a certain person or organization that the metadata is about.
The Boston bombers weren't caught because of cellphone data. They were caught because they engaged the police in a gunfight; one of the brothers escaped, and he was caught because a man reported that his boat had a blood trail on it.
They actually were tracked by the cell phone of the man whose car they hijacked. Obviously, this was only a small part of the story and isn't the same kind of cellphone data.
The Boston bombers weren't caught because of cellphone data. They were caught because they engaged the police in a gunfight; one of the brothers escaped, and he was caught because a man reported that his boat had a blood trail on it.