If you are a ben Torah, knowledge of Yiddish is highly useful. Most of the advanced world-class (Yeshivish or Chassidish) Shiurim available today are in Yiddish. Most yungerleit are learning in a blend of Yiddish, Aramaic & English (#YeshivisheReid).
To be in the Yeshivish system without knowing Yiddish puts you at a disadvantage. You won’t understand the Shiurim, you won’t understand the Shmuzen.
When I started Yeshiva, my rudimentary Yiddish was enough to tide me through; I could listen and understand most Shiurim (the shmuzen were more of a challenge at the beginning), but I couldn’t speak it. I was zoiche to listen to chaburos from a fantastic Talmid Chochom, back when I started Yeshiva. I used to “ding in” in English, and the Maggid Shiur couldn’t understand a word. In the beginning, I used to ask the guy next to me and he would ask for me; that doesn’t work if you’re going to continue an interactive discussion. Eventually, I would write down my questions and break my teeth in Yiddish afterwards, in private, when it was less embarrassing.
In the Yeshiva I am in now, most of the hierarchy either don’t or won’t speak English. If you want to discuss a sugya with the Maggidei Shiur or Rosh Yeshiva, you have to discuss in Yiddish. And if you can’t, you’re at a major disadvantage.