Well, I rather prefer to let the speaker speak for themselves, so I went and looked at the
original article. If you go and read what Ruth actually says, they're concerned about YA literature replacing literary fiction in adult reader's lives. They are concerned, for example, about the relatively uncritical portrayals life that don't capture the depth of more adult/mature understanding of the world; they're concerned about the simplistic endings in YA literature compared to more emotionally and morally ambiguous endings of adult literature; they're concerned about people gravitating solely to pleasurable "feel good" literature instead of more complex storytelling. Though I don't agree with moralizing the reading of literature and suggesting people who read predominantly YA should be "ashamed" of themselves, I don't disagree with Ruth's point. The content and diction of YA literature is pretty vacuous, comparatively. Some is better than others, but the core of what Ruth is saying is correct: YA literature is not a substitute for adult literature.
However, Ruth doesn't make the point against YA literature that, in my mind, is the far more substantial and valid criticism of adults reading YA literature. Sure, YA literature tends to be more simplistic, but so is an awful lot of mass-market adult literature. What really sets YA literature apart is diction: simple vocabulary and structuring that is easy to follow for people whose grasp of the English language is still in development. If adults are reading YA literature because they never managed to move past an eight-grade reading level, that's a problem. And while I didn't agree with moralizing the reading of YA literature for its own sake, I definitely do
not disagree with moralizing the reading of YA literature because you're poorly educated. Or, as the case unfortunately is, because our culture has been fostering a pervasive dumbing down of language. Also, I think, a problem.