For me it is all about motive. Some goon on X Factor massacres a classic and you know there is no soul in it and that the only motive is cash. But then you get covers that are done from the heart - maybe as a bona-fide tribute or in some cases because the artist in question knew that they could add something to it. Look at all those old blues covers that the Stones did back in the 60s, they were all authentic and no one can decry them for doing it.
The Ramones tribute "We're A Happy Family" is a great example of some huge bands that honestly wanted to say thanks. Take Kiss doing Rock N' Roll Radio, hell it is probably better than the original (forgive this apparent heresy) as they reworked it and made it their own.