well by and large they can. the point about auto tune is it is used unnecessarily - it's contributing to an attitude that deems things "out of tune" that would previously have been deemed character, style, individuality.

I was listening to some Shirley Collins at work yesterday (Harvest Years anthology). She's a singer who goes a bit flat on occasion: her pitching occasionally wavers. That doesn't in any way spoil my enjoyment of her music; I don't even regard it as a fault, I just regard it as part of the experience of listening to her.

If the Stones albums Their Satanic Majesties Request or Beggars Banquet were submitted to the record company today as new albums, the record company would tell them to re-record it and, doubtless, would have suggested they use auto-tune: there's some really sloppy timing on it and absolutely tons of off notes. That's why they are such good albums.