The MQTT Action can only publish to topics.
publish(String brokerName, String topic, String message) : Publish the message to topic using the specified MQTT broker.
You must use the MQTT Binding and Items to subscribe to topics. And that makes sense. How would one write a Rule that subscribes to a topic?
It isn’t that we can’t just create the Thing Channels (and Items to link to them), but that in many situations it can be significantly more work.
There are situations where one can write 2-3 lines of Rules code to handle ten or more topics verses still needing at least one line of Rules code plus all the work to create ten or more Things/Channel.
I don’t actually use the MQTT Action myself so I may not be the best person to drive this discussion, but I know that Vincent and many others do to create code like I described. And this feature will be something that will be lost in the move to MQTT 2.0.