"Contact me via mail or XMPP at <single address>" is IMHO one of the best parts of XMPP.

I find it quite awkward to tell people "oh, by mail contact me at address1, if you use Matrix, then you need address2, when you have Signal use my phone number at <number> and when you want to have a meeting with me use that URL at address3!"

Nonsens! Mail, XMPP and SIP enables you to be reachable by a single address. IMHO a big plus!