When I used Word, I used to run the spellcheck mostly to find and eliminate passive sentences, and to check those stats, because I didn't want them to get very high.
IMHO, in fiction, if you want a reader to follow the storyline and characters, it's really important to be easy to read.
But.. having said that, I kind of think it's important whatever you're writing. I mean "A Brief History of Time" is a great book because it's easy to read and comprehend, and so it lets the reader in. Instead of wading through it, you can relax and let Hawking tell you cool secrets about amazing things.
There are exceptions. Out of curiosity I checked a website to see what some classic authors score like (some of Oscar Wilde's are quite low), and Walt Whitman's Leaves of Grass was at the top, but that's different, because it's like reading someone's soul. Robin Williams springs to mind here, ranting about measuring poetry. Whitman's score is a failure of the equation.
Wow.... that's a startling conclusion. Maybe I should always have aimed high.