The point of my comment is that it's perfectly fine without opt-in.
A discussion that never goes beyond "I (dis)agree" isn't much of a discussion.
1. This was not a feature that I originally signed up for, and increases the possibility of irritating/trivial items to be sent to me, I have a spam filter for things like this and in general would rather not mark google as spam, they send me a good number of other things which I do find useful (which seems to be an abuse of the "trust" however thin that I have with them).
2. My private email is perceived as somewhat sacrosanct even though it is an obvious federated system. A somewhat apt analogy is my home address, while some people could definitely find it through various means and communication happens all the time by parties I do not want to send me mail, I do not give it nor do I invite strangers to approach it, and I especially dont like it when a friend of a friend gives out my address to strangers.
3. In general, opt-out features of social networks are by design an anti-feature meant to drive more engagement for them, and has little to no perceived benefit to me. (Yes you can say facebook does it, and that is why I dont have a facebook account)
Better?