I don't read fiction largely because I don't like its deceptive nature, but I do fall prey to it through different media (e.g., watching movies and television shows). I have a lot of friends who only read fiction. It's their love in life. Let me tell you why I think it's dangerous to read only fiction without a healthy dose of non-fiction.
I think it's dangerous due to one single factor, and that is because it seeks, not only to entertain, but to communicate ideas in a non-critical manner. In other words, it wants to convince you of something without allowing you to think through it. It simply wants you to have an emotional tie with the idea, so you are more likely to have sympathy toward it, and in reality, adopt the idea without knowing critically why you adopted it. When combined with our culture's experience-oriented approach to beliefs, the vehicle through which fiction seeks to persuade, i.e., through vicarious experience, almost guarantees a large following of whatever ideas its peddling.
For this reason, fiction has been the primary means through which all sorts of false ideas have influenced our culture, and have caused it to lack in critical thinking skills. When asked why someone believes an idea, most people cannot tell you why. They just believe it. That idea wasn't necessarily put there by the fiction writer. It likely had a philosophical base to it. But it was made the most palatable through fiction. Where one might have rejected it as a bad idea if he or she was given the chance to think about it critically, it will be adopted wholesale now because of the emotional inroads it has made into the person's experience. So, for instance, forget that Dan Brown's history is completely fabricated and a joke, you'll have thousands, if not millions, of people believing it because they never bothered to pick up an actual, non-fiction, history book.
This is why a heavy dose of fiction without an equally heavy does (or greater dose) of non-fiction is bad, even if the ideas within fiction are good. People need to know why they believe what they believe. They need to think critically. Our culture has been heavily influenced by a legion of erroneous, and therefore harmful, ideas upon which it now lives out its daily life. Yet, no one knows why they believe them. They just do. In that respect, fiction has become a powerful ally for chaos, since even if the ideas are good, good ideas are easily assailed by bad ones when not supported critically. All a bad idea has to do is come to an uncritical person and convince them cognitively of it, and that person who once believed what was good will be quickly swept away.
Don't get me wrong. I don't think fiction is all bad. Notice, my distinction is between a heavy dose of fiction in one's life absent of an equally, or larger, dose of non-fiction that would give one critical thinking skills toward a particular subject that fiction might present. I'm just tired of people being deceived and not knowing why they are. I'm tired of the "well, that's just what I believe" nonsense. You should know why you believe what you believe. Where did you get that idea? Why is it more valid than the alternatives? What source of authority are you trusting for this information? What are the beliefs one must hold first in order to come to this or that particular idea or practice? If you can do that already, then have your fill of fiction; but if you can't, I suggest dropping the Twilight book in your hand, turning off "Glee," skipping "Angels and Demons," and go take a stroll through the non-fiction section of the bookstore for awhile. Find books that will restore to you the thinking skills God gave you, and start answering that question, Why do you believe X? with a series of well thought out arguments. You might just then find that the fiction you read becomes even more stimulating than it was before.