Its legal until the follower complains/reports your messages. As you said, its unethical but you can still find ways to attract their followers rather than just private messaging them.
Yes, I completely agree with you mate. All our end-users need the CONTENT the most they wanted to read. If we can satisfy every single user of our site with our content, that is really great enough to stick our organic rankings to the top. As you said, if decent link building combines with it...
@Zipix and @MBS, Thanks for the useful comments especially the UX one. Can we be discussing something which is not common and new? HQ content and quality link building is always recommended. Please share more things you know which would work in coming years.
Thanks again.
It is okay mate. It would also mean to Root Domain If the topic was related to it, but here the topic is about metrics, so I just made sure it means Referring Domains. .