Yiddish is useless for anyone. There is not reason anyone should learn it.
If you want to live in the USA, Learn English, There is no reason any jew should not learn english. Unfortunatly My great-grandfather never could learn english so I could not speak to him. I seriously doubt my mother has spoken yiddish in decades and it was her first language.
And frankly after seeing people here talk about it, I made it my business to listen more carefully to people who speak it now , and I understand it better than I realized. They do not speak proper yiddish today. Listen to proper yiddish and listen to what is spoken today and its not yiddish. I dont know what it is , but its not yiddish.
I have been to Paris, and I met some jews there who did not speak English, They were SEPHARDIM and they spoke Hebrew, I spoke to them just fine. If I really need to speak to another jew in a common laguage, More jews speak Hebrew than speak Yiddish, and in fact more jews speak ENGLISH than speak Yiddish.