It's one of those conical pieces of writerly advice: "don't abuse the thesaurus." The problem being that you find words that are unnatural and discordant with your tone and style, and they stick out, and it sounds bad and reads worse.
In a similar vein I remember someone quoted Heinlein as saying that always wrote adult novels using the simplest language he could think of, for accessibility reasons, but in young adult novels, or stories pitched to the YA audience, he would occasionally throw in a hard word, because kids liked feeling like they were being trusted to be smart enough to handle complicated things.
So I'll admit that I occasionally use a thesaurus. More often than not, it's to remember a simpler word. Like, in something I'm currently writing, I just looked up "concern" to find "interest," because I was having something of fixation, and the reference was good to get me unstuck. We can all imagine, of course, how this could have gone awry.