[ge0rg]

or at least RFC 3921

RFC 3921 was superceded by RFC 6121 in 2011. You might want to update your facts before you criticize the protocol.

The message routing rules leave many things to the server implementation, adding a bit of fuzzyness, but all of this has been addressed in Carbons, and really, in the last years there was significant progress in how mobile XMPP works and feels.

as soon as I want to have a three-way conversation we have to pick a server and a room name and...

The modern mobile XMPP apps like Conversations already have a seamless 1:1 to group-chat upgrade mechanism, and people are working on writing down according specifications.