That main reason doesn't apply only to zeromq, though; with google's protobuf you can write an rpc layer on top of pretty much any channel in ten or twenty lines of code.
Sure but with zeromq we get builtin load balancing and pub/sub.
ZeroMQ is not a serialization or RPC mechanism, but a smart routing system for packets of data. It doesn't even include one. Many people use google's protobuf with ZeroMQ.