The fact that someone is a native is by no means a guarantee that he/she speaks (and especially writes) a better English that a non-native (especially a well-educated one).

I would like to add one curiosity to native / non-native English benefits: some agencies actually prefer proficient non-natives to write for them, because the English of non-native writers uses less jargon and only locally understandable jokes/references. Thus, on a global level, a proficient non-native is often even more understandable than a proficient native.