I think that so many people advise writers against using the thesaurus because you can quickly turn your book into purple prose with the “Ah! THERE’S a word no one uses” mentality. However, I use the thesaurus constantly. I think of a word and know it’s not quite what I want and the thesaurus jogs my memory. Like just now. I knew I didn’t want insignificant and I really didn’t want imperceptible. What I wanted was subtle. (I could have gone the purple prose route and chosen evanescent but I generally like to keep things simple.)
And that brings up another thing that at least some people don’t like: “library words” in books. Because readers don’t want to be bothered with looking up an unfamiliar word. Now, that’s just silly. There’s a difference between extravagant words and words the reader hasn’t encountered since high school or college. Yes, I will throw in a “library word” if I feel it’s appropriate to the sentence and the narrator. Because going to the dictionary is good for all of us.