Erlang is definitely worth learning but definitely not required by Elixir.
Learning the concepts that embody Erlang such as tail recursion and function matching, make Erlang worth learning. Erlang is also a special mix of Prolog and Lisp.
To add to this, Erlang is a very simple language that you can learn in just a few days. I don't know Elixir but I assume that, as is the case with Erlang, the more difficult thing to learn is OTP and not the language itself.
For me Erlang was worth learning because it's so interestingly different to many other languages because of its Prolog roots.
Learning the concepts that embody Erlang such as tail recursion and function matching, make Erlang worth learning. Erlang is also a special mix of Prolog and Lisp.