Most people don't become polyglots either, though, so that's not a good measure of the utility. It's not a question of language-learners learning etymology would be useful to the man on the Chatham bus, it's a question of whether it is useful to language-learners.

Knowing the etymology of, e.g., "goodbye" makes it click faster/not be weird when you learn, e.g, that "hello" is "Dia is Muire duit" (God and Mary be with you) in Irish Gaelic.