Depending on who you ask, if I were the director of the central bank then my answer would be "mandatory". This question has long been answered by the fact how governments want to control as much as possible, and prohibition is the ultimate judgment of their inability.

The right question is not about necessary or not (globally) as it is an option individually. If you still enjoy the facilities of a centralized financial system, it shouldn't be a problem even if you are controlled and bitcoin is not for you.