Hell, I like Mozart and other classical music, folk music from various countries (particularly fond of Turkish stuff), Metallica, Dragon Force, N*Sync, Linkin Park, some dubstep, Evanscence (though while I like her voice, I'm not a fan of Amy Lee) and if you forced me to listen to Bieber's albums, I'd probably find at least one song I like there, too.
Some people have different tastes in music. Some aren't always high class. Some, like me, have exceedingly wide palates where we can listen to pretty much any genre and enjoy it as long as there's something to enjoy. It's like enjoying potato chips and filet mignon. It happens.
That doesn't mean I have to like the singers, though. Nicki Minaj wears so much make up that it's hard to believe she's not a computer generated character. Bieber is a molting peacock. And as I mentioned before, I don't like Amy Lee, just her voice. The fact that there haven't been any new Evanescence albums (that I know of anyways) since she announced that she is Evanescence and (I think) caused her best friend to basically leave the band out of disgust tells me that she basically screwed herself over in that regard.
So, I don't think Bieber and Nicki Minaj are "valueless", but they do have value for the wrong reasons.
Also I like the use of autotune as an "instrumental" effect, not as an "audio correction" effect. That is, using autotune for autotune's sake instead of using autotune to fix someone's singing.