I'm only passingly familiar with either, but it seems like most of what ZeroMQ offers is already built into Erlang, yet there's an Erlang-ZeroMQ binding that's fairly active according to its github page. What am I missing?
ZeroMQ bindings are available for a lot of programming languages, not just Erlang; this is one of the library's great strengths.
Erlang's built-in mechanisms are probably great as long as everything uses Erlang, but I doubt they are in any way compatible with other languages' native networking mechanisms.
Perhaps communication with a ZeroMQ bound but non Erlang process.