It would be the most practical way for a DBA to monitor an application. But it's not going to be convenient for clients like pgadmin or psql. Even a web server may want to, for example, stream ajax code updating a progress bar until it has results and then stream the ajax to display the results. Having to get the backend pid before your query and then open a second database connection to monitor your first connection would be extra footwork for nothing.
But that said, it CAN be coded and work just fine no? ---------------------------(end of broadcast)--------------------------- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match