1) Bear in mind that The Diamond Age takes place in a future society that has adopted the manners and mores of Victorian England. Their version of English is known for being convoluted, highly-educated, and including many terms that have since fallen out of general use. The unusual language isn't just Stephenson showing off; it is evocative of the culture he's writing about. Examples of these words include singular, scrum, and foetid (American spelling: the unassuming fetid).
2) Many of those words are worth learning. They concisely express useful concepts which would otherwise take some trouble to get across. I'm sure I'm more intellectual than most, but my ordinary communications would be genuinely compromised if I didn't have the words ecumenical, rote, or façade.