1. True, jerks are jerks and will be no matter what we do, but denying them one more thing to superficially bitch about is a good thing.
2. I tank too. I don't want people ALSO bitching about the DPS I'm doing/not doing in addition to how I don't use cooldowns and/or should pull more stuff. I imagine a parser would also be able to tell you how much the Healer is overhealing or not contributing to DPS, that sounds like extra unfun they don't need. Saying that those classes suffer so DPS should too isn't a great argument. You'd be adding grief to everyone's life if this couldn't be turned off.
I think it's very important to provide the functionality everyone is asking for without auto-opting in everyone.