This is all just a mess that I'm not sure what to think.
Let's just say, for the sake of reference, that Rehtaeh was raped (I don't know enough about the case to make an opinion on this), then the lack of anything having been done about it does seem like a travesty. Still, I'm not sure getting Anonymous involved is a good idea. This group of Anonymous may have said that they won't dispatch vigilante justice, but considering the veiled threat at the end of their statement it would be safe to say that this is an operation that can easily fuck up. Perhaps the other parts of Anonymous won't be as reserved and may go farther than they should, and considering how easily someone innocent may accidentally be identified as one of the rapists, this seems to me as something with no real happy ending.
Further, let's say this is a situation not only where only Anonymous can do good, and manage to actually do everything by the book. I still see their inclusion as opening the flood gates as it were. Alright, so a group came together to solve a case that the justice system was unable to do. What happens next time though? Does the group wait after nothing happens, or do they start to act before anything can even be done? What if everything has been followed to the letter of the law, but the group isn't convinced and continues investigating? This seems to me like inviting trouble, trouble that we may be happy to have now considering the circumstances, but trouble we'd have a hard time getting rid of once our emotions die down.