An author has to think about who will be reading what he/she writes. Authors that work professionally sell to particular audiences that they understand well. They know what that audience whats to read and what that audience doesn't want to read.
It's kind of an elementary rule of writing: know your audience.
Authors get in trouble when they end up writing to the wrong audience.
So in a lot of ways, content ratings would help authors understand their audience better.
I would say that visual mediums are more likely to stick in one's mind. Music, while not visual, also has tremendous ability to stick in one's mind. How much prose do you have memorized, compared to how much music you have memorized?
Memorization works through three processes: repetition, emotional context, and connection to previously stored memory.
There is an argument that music or visuals help memorization, but that is yet to be proven. Memory is linked to cultural factors as well as individual learning styles and is greatly influenced by developmental and environmental effects. Personality may also play a role. So what sticks in one's mind is highly subjective.
One of my big arguments for rating systems comes from working with adults. Many adults who have been convicted of sexual crimes are aroused more quickly by music, visuals, or words that are suggestive than an unconvicted person (usually). When working with adults who suffer from sexual addictions it is important to help those adults gain control of their environment so they are less likely to commit other offenses. Content ratings can greatly improve the livelyhood for these individuals as they learn to more responsible. ANd for some content ratings will probably encourage worse behavior.
Basically, there is no base-line in human behavior to what a person (child or adult) can handle. Some people are very resilient and others are very fragile. There is an average and, of course, lots of outliers.