The MQTT Eventstream is a very powerful tool which makes it possible to connect several instances of Hass together. However, the current function of the Eventstream only allows you to send out ALL events to the Eventstream which makes the subscribing instance identical to the publishing instance.
This is not preferred if you only want to have certain functionality on the subscriber. One example where this could be useful is if you want to have a node dedicated for Z-Wave, Rfxtrx or perhaps HDMI CEC, separated from your main instance and you don’t want to put all the load from the main instance on a smaller node.
If it were possible to choose which domains to include and/or exclude from the Eventstream it would be very easy to setup nodes dedicated for certain functionality.