When the NC-17 was created in 1990, it was intended to be a rating to avoid situations just like the one faced by Blue Valentine. It was a substitution for the 'X' rating, which had been exploited by pornographic filmmakers to such an extent that 'X' was shorthand for out-and-out pornography. NC-17 was intended to be a new start, a rating that indeed classified a film as 'for adults only, no exceptions', while differentiating said films from the likes of Deep Throat or Debbie Does Dallas. The first film to receive the rating, Henry and June, actually received an Oscar nomination for Best Cinematography. It was not a massive hit, but it wasn't the kind of film that usually breaks out of the arthouse anyway. But then, the theater chains, the networks, and newspapers seemingly ruined the game plan.
Newspapers more or less unanimously stated that they wouldn't carry advertising for NC-17 pictures. Major television networks refused to sell advertising to NC-17 films, and many major theater chains refused to book them for exhibition. And just like that, NC-17 was indeed the new 'X'. With the exception of Showgirls, which ironically represented what everyone wrongly presumed about the NC-17 rating, films slapped with said label did not open in major theaters or participate in mainstream advertising campaigns. It wasn't supposed to be that way. But it is. Regardless of what we think of this MPAA decision or that MPAA decision, the fact of the matter is that if theaters would carry these pictures and media outlets would readily accept advertising for them, this would all be a moot point.
We can argue that, in America, there shouldn't be any rating that prevents me as a parent from taking my child into a theater to view any movie whatsoever alongside me, but that's an argument for another day. But the problem with the NC-17 is not the fashion in which it is awarded, but the manner in which NC-17 films are distributed and marketed. If theater chains like Regal or AMC would screen NC-17 films just as they do R-rated pictures, than this would all be an irrelevant debate.