There are numerous programs with leveled text that can be used for instruction, and I see some value in that. For a student to reach frustration level because they cannot understand what they are reading is counterproductive. But, I do not agree that a student will not learn or be challenged from reading a book that is "too low". While lexiles take in such things as vocabulary and text complexity, there's no true way to measure the depth of emotion and meaning a book can bring to a student. I cringe when I hear of demands to "level a library".
I will never forget an avid reader, deeply empathetic toward others, very mature for a fifth grader, who came to our library and asked for suggestions. I booktalked Ruta Sepetys' moving book Between Shades of Gray to her, only to have a teacher (no longer at our school) deny her the book. Why? Not because it deals with a very serious subject, Stalin's genocide. No, it was "too low". Yes, we have students who like to carry around Harry Potter or other large books who will probably not finish them, but they should get to choose at least one book with no restrictions. Maze Runner, with a lexile level of 770, Hunger Games at 810, and Wonder at 790 would be off limits for many of our students, but Diary of a Wimpy Kid, several of them at 1000+, are more acceptable. While Jeff Kinney's books have turned many into readers, books at lower levels can still provoke thoughtful dialogue and encourage students to read more.
As adults, do we always read "at our level"? Does my book club consult lexile levels? Do I ever push myself to read something that is a bit more complex if I like the topic? Girl on a Train comes in at 760, All the Light We Cannot See at 880.
We also must realize that for years, publishers were under pressure to publish many "Hi-Lo" books, meaning high interest, low vocabulary. Then, in particular, the Common Core Standards came around with the accompanying push for higher lexiles, especially in nonfiction. I have done numerous searches for books at higher lexiles, with many of them being dry as dust or not appropriate developmentally for middle school students. Yes, the publishers are now touting all kinds of correlations to CCSS and more books appear to have some higher levels, but they still do not appeal to many students.
The motto of Banned Books Week is "Celebrating the Freedom to Read". I'm all for letting students read what they are truly interested in. As with any skill, we improve with practice. Why not make that practice enjoyable, rather than a burden?