You don't need to use Google Talk or have a gmail address to use XMPP. It's just another implementation of an existing standard.
I don't think that I need handholding from a protocol to the extent that it saves me from putting their chat address in my address book. If I don't know it, I can text them and ask:)
I don't ask their home number to automatically give me their work number.
It seems all that iMessage really does is enable users to discover connectivity through an alternative means and enable communication through that means. There's no reason you couldn't use the same mechanism to discover a JID/XMPP ID.
I don't think that I need handholding from a protocol to the extent that it saves me from putting their chat address in my address book. If I don't know it, I can text them and ask:)
I don't ask their home number to automatically give me their work number.