@oratio, mostly but not exclusively in writing. In speech, it depends on who I’m talking to and the level of the conversation. The word has to fit in smoothly or it shouldn’t be used. It would never be “Hey, you know, that was, like, really inchoate, dude. Defenestrate me if I’m not totally chthonic about it.”
@gailcalled, I think it is indecent to use a showy word such as phthistic more than once in an entire book unless it is a key to a story element such as setting, plot, or character (“ox-eyed Hera,” “the wine-dark sea”) or it is bonded to your thought process (e.g., blasphemous, for Lovecraft). For any word that stands out and calls attention to itself, twice is too much and maybe even once is too much. If it sounds like you’re reaching for it, the strain will show. Naturalness is essential. One who does this beautifully is Michael Chabon.
My apologies to the OP; we have strayed terribly.