I think that the reason why tarnishment is a valid thing is because most consumers say to themselves: "Oh, how could Company X let Company Y use their name so immorally?" without considering for a moment that this is either an unreasonable expectation on Company X, or an unreasonable expectation on everybody else.
It seems to me to be a consumer confusion problem, but one that's been promoted by these big companies.
What would TWC do with free fiber, government employees, buildings, and discounted services? Would they make service better and cheaper for their users?
I think not. I have a feeling that if TWC/AT&T got these perks, they might use them to develop faster services, but would charge the users a premium to get access to them; or do nothing with them, since TWC does cable, and AT&T does DSL, letting the users wait for Verizon to come in with fiber, so they can continue to have tiered services that technically (but don't actually) compete with one another.
I used to be a big fan of the NC, until I realized that I was only using it to prevent other people from making money from something I did. And then I realized that that made me just as bad as all the other culture cops out there, and since I didn't care if people copied my stuff, why should I care if they profit off of it? Ultimately, what I wanted was the credit for the parent work, and as long as there's a CC-BY, I'm still good.
I still do SA, though, as a matter of principal.
That being said, I think that NC and ND are good things to have to promote openness in otherwise gated content. I think if CC did away with these clauses, they would get fewer new adopters. The better solution is to keep these clauses around to encourage growth, but to educate people (as I was educated) that while it might be comforting to have the NC/ND clauses, they should only be used as a gateway to better access to content.
Also, if the CC got rid of NC, Cory Doctorow's new books wouldn't be released under CC-4.0 licenses due to an agreement he has with his publisher.