Good writing isn’t about impressing people with big or little-known words. It’s about communicating. Short and simple always communicates with more power than long and complex.
Take that last paragraph. I could have written: Effective written communication eschews contriving to flaunt one’s abundant lexicon. A succinct, elementary endeavor invariably fosters more forceful intercourse than prolixity and intricacy.
Do not fall for the arrogant saw that you must aim at an eighth grade reading level. I don’t even know how to judge that, and neither do you. Simply aim for getting concepts into heads with as little work as possible on the part of the reader. This has nothing to do with how smart the reader is. It has to do with the fact that even the stuffiest Ph.D. would rather glide through an easy read than slog through a needlessly challenging one.
Using a thesaurus entails a couple of other risks. There is a danger of not quite getting the use of the wrong right. A thesaurus may suggest disingenuous instead of sneaky, but they are not quite the same thing. There is also a danger of inserting a word that is out of character from its surrounding text. For instance, the dog barked at the burglar works a little better than the dog vocalized at the burglar. For both reasons, my 10th grade Honors English teacher drilled into us 10 vocabulary words per week, yet warned when giving us a writing assignment, “If you use so much as one vocabulary word, you will receive an automatic F.” His name was Harry Walker, and he was a great teacher. If you went to Reno High way back when, you knew him, or at least of him.
There is nothing wrong with growing and using your vocabulary. But for writing, seek the best word, not the showiest. And don’t try to use words outside your normal reach.
There is one legitimate use for a thesaurus: when the right word is on the tip of your tongue, but you can’t quite get to it. In that case, by all means grab a thesaurus and hunt it down.
(I can’t resist adding this anecdote, written by a former bookstore employee, that appeared in Writer”s Digest some 30 years ago. A customer told the author that her son needed a specific dinosaur book for school. Of course, you have already guessed the outcome. After searching the Natural History section, the employee realized that the child had requested a thesaurus.)