[zaik]

If a service requires my phone number, it is indeed a privacy concern. Phone numbers are strong identifiers; for most people they are tightly coupled to their real world identity. Most XMPP servers do not even require an email address (+ I have the freedom to host my own and still be able to communicate with everyone else using XMPP). Thus I can create a new XMPP account without linking to my real world identity.

[infosechandbook]

Excellent example for what we mean: Our article doesn't discuss anything about "What is needed to register for service X." Discussing this is perfectly valid; however, it isn't about the article.

[zaik]

It is about the article if its conclusion is 'we recommend to use X instead of Y'.