I never knew Roget's was organized like that and I have had several different editions over the years. I'm one of those people who always used it as a compendium of synonyms.
A lot of books that call themselves Roget’s have abandoned the dual structure (classification-index) in favor of a single index structure built like a dictionary, basically constructed for people who want a different word than what they would normally use, as mjd writes, rather than people who are interested in the store-house (thesaurus) of the English language.