Doubtless few people today who have the slightest thought of, well, a passing need for a thesaurus--a book providing synonyms--have ever opened Roget's. With Thesaurus.com, and all of the other quick-fix solutions to vocabulary issues, something as extensive and, yes, requiring actual research to use, does not fit into the "texting," "Tweeting," "blogging" landscape most people choose to traverse in the latter 20th and early 21st centuries.
In reality, few ever DID have such a need, as the phenomenon of "universal education," actually a true chimera, is a modern anomaly. The truly educated--knowledgeable in literature, history, geography, and the sciences (not the post-Darwinian "everything is relative" excuse for the scientific method--has always, and always will be, a small minority, as most human beings much prefer to be told what to believe, and when to act upon it.
That being said, Roget's Thesaurus is an absolutely invaluable tool for the serious wordsmith, the dedicated scrivener. If one wishes to truly say or write what one wishes to communicate, many, many more words than exist in the realm of common communications must be utilised, and, as Shakespeare put it, "therein lies the rub," for VERY few wish to take the pains to say precisely what they are thinking or mean to convey. For tome raiders, authors, editors, and serious writers of any sort, Roget's is an absolute must.