In a review of A History of Roget's Thesaurus, John Whale (nice name) says, "The safest storehouse for writers to fetch words from is their own head." He does concede that
Of course a thesaurus can help a failing memory recall an elusive word, even though reliance on a thesaurus could end in making more and more words elusive. It can help non-native users of a language enlarge their vocabulary, though . . . with a risk of solecism. It can help copy-editors clarify a paragraph where the writer has used the same word in two different senses. It can ease the fitting of headlines into a fixed space, or the writing of verse in a regular metre, or the solving of uncomplicated crossword clues. Perhaps those are the aims that Roget is still in print to serve.
In my experience, a thesaurus is a machine by which students convert a few syllables to many and an exact meaning for one that grazes the mark.
I once had a student who wouldn't stop using qua until I told him I would reduce his grade by one letter every time I found it.