@Fish - Very true!
I would like to add that reading-level as per the calculator does not necessarily reflect insight-level. It's not that hard to learn how to write in a way that boosts the "test score" artificially without adding any insight (see, for example, almost ever single essay I wrote in high school). Such behavior can be fashionable. For example, current titles and urls tend towards short and even misspelled (e.g. bing.com WTF?!) whereas Victorian titles tended towards entire paragraphs (e.g. A discourse on available research based on certain words and phrases and delivered in list from via a cleverly designed contraption dot com Double-WTF?). Here's a complaint against writers of the latter kind: http://www.orwell.ru/library/essays/pol ... h/e_polit/
I think Einstein's observation about theories also holds for writing. It should be exactly as complex as the underlying thought but no more complex. It's quite easy for a writer to err towards "too complex" to the frustration of the reader. Distilling words is not easy. It is however extremely pleasurable. To a writer, good prose induces the same feeling as good source code does to a programmer. It's beautiful but a lot of work.
In sense of *free* online calculators, reading-level has nothing to do with writing ... but rather reflects the required ability from a reader's to feed themselves intellectually from a diet of either steak (requires lots of chewing and good stomach acid), hamburger, or blended babyfood that's 90% steamed carrots, 8% liquid smoke, and 2% "meat product". Unfortunately, modern readers have become so used to reading the word equivalent of babymush (the professional standard for a mass-published work of non-fiction is a 6th-grade reading level) that anything more incurs the intellectual equivalent of heart burn and acid reflux.
There's now an expectation that any expert on any subject will be able to provide a complete "mind meld" in the form of a babyfood sampler. If not, it's "too complicated", "too much theory", "super dry", "too complex", "poorly written", "forcing you to listen to all the arguments", wah wah wah!
, blame the writer. Oh the humanity! Current sentiments have err'ed towards the expectation that experts must put their "best" arguments in a blender and serve the result in a way so it can be ingested through a straw or they'll get a poor review on Yelp! Bah!
In the present regime where even non-fiction books get edited down to a 6-grade reading level, there are additional problems. Reading level is calculated via some formula of average word and sentence lengths and afair doesn't consider the reader's vocabulary nor the reader's familiarity with various concepts. It's possible to induce heavy imagery and deep insight at a sixth grade reading level (see Hemingway or a mathematics paper) and it's possible to do the opposite (see a legalese). Because so few read books these days (extreme Pareto-law ... especially when corrected for Harry Potter and Hunger Games sales), the loss of mental models also means that any writer on complexity faces additional problems and would have to err towards pop-culture references (Matrix movie) rather than classical literature (Plato or metaphysics).
Edit: brute beat me to it and with less verbiage than my Victorian version @brute - This problem could be well-illustrated using a 2D-graph which would be shorter still