I think you and a few others are unclear on what I'm talking about.
In order to let people know it (private/public message notifications - NOT marketing emails) exists, I am/was going to set everybody's to ON.
Everybody sees the emails, they know about the feature now. If you don't like it, it's literally two clicks to turn it off and it stays off. (Via Tools > My Settings OR the link in the emails)
The only thing I'm talking about is the initial setting for accounts.