XMBC web interface just streams to a different client. Here, we have a client requesting to stream to another client, and synchronised by the server. The key part is the synchronisation between multiple clients.

The patent also deals with a few other types of concurrent streams for other applications, beyond what Chromecast does.

Come to think of it, if you use Firefox on mobile to access YouTube, then “send tab to other device”, and send it to a desktop computer connected to a big screen, it could be interpreted as violating the patent as it’s using Mozilla’s “back end server” to relay the message

That may well also violate the patent. It would likely depend on whether the devices are synchronised, or if the desktop is just getting a link and streaming separately.

Just because lots of people use it without paying doesn't make a patent invalid. You only have to look at what happened with the patent for WiFi.