Earlier quoted context omitted.
> […] in a world where clients are all free and the identifier of choice is the phone number […] To me this translates to: “Please run our proprietary software to participate and use a high-value nearly unique identifier to identify yourself — we really like to know that john.doe@gmail.com is the same person as feetfetish33 so we can better quantify you for targeted advertising.” I am willing to entertain the notion…
I have yet to encounter a chat network that uses targeted advertising, so you're well off into the realm of conspiracy theories there. Chat networks are all moving to the use of phone numbers because users mobile phonebooks are a vendor neutral, open access social network of high value contacts that almost everyone has and for which there are simple APIs available. Phone numbers have other advantages. They are diffic…
You won't see that because that is not what makes the phone number so useful. Its biggest benefit is being able to correlate all those separate data profiles via one unique key. This makes these profiles more valuable to advertisers. You won't see advertisements in most of these apps (it would drive users to the competition) but they don't have to; they just show them in the browser. They already know it is you thanks to the ubiquitous social media buttons and tracking going on.
This is largely conjecture, sure, but the fact that using your phone number is a requirement rather than an option makes me suspicious.
Sometimes I don't mind if two services know that I am the same person, but I would like to be able to choose for myself. Sometimes I do like to use a throw-away email address and a VPN to use some service, simply because I care about my privacy. Often I don't feel that a service needs to know who I am beyond an alias.