Or can you give me one good, solid reason why an ordinary person would want to use a non-Google XMPP server?
Some employers provide on-site supported XMPP servers. Until recently, I've been able to use ours to collaborate with external partners on GTalk, using federation.
Some vendors provide built-in XMPP servers as part of other products. I'm aware of one telephony platform that does so and one IT helpdesk service that does so. Using their servers enables certain useful features, like "they look like text messages to phone users" and "customers can open issues with the help desk by sending them IMs". Those are more useful when federation works.
Some web service providers have XMPP support in their service platforms. I used to be able to have IFTTT send messages to me, due to federation. Google's announcement about turning off the service has caused them to remove the channel from their service entirely. Now I can't use it anymore.
(Those are just the ones that have been impacting me personally as of late. I'm sure others could think of more. No, they're not mainstream. Yes, they're real.)