The point is that broadcasting the same message verbatim across every medium and community. removes one's ability to tailor the message for the standards of that community.
Not every message needs to be tailored. And not every message need be the same across every medium. Having a write API doesn't force anyone to do either of those things. It would not give or remove anyone's ability to tailor as needed or broadcast verbatim as desired. Having a write API would allow 3rd party apps to add G+ to your options but doesn't force you to act any way you would not already act without the write API. My point was that having a write API doesn't have to be bad... but some abusers will ruin it for the rest.