Hi Bart,
Status messages aren't passed through the flow. They are indeed mainly for visual effect, BUT... they also create an event that the status node receives (and can filter on). This is similar to the Catch node that catches node.error events.
Status is intended to reflect the status of the flow (or node) itself - rather than the state of the message - though this is blurred :-)... So it is intended for things like the state of a network connection, the depth of a queue, if a node is active or inactive, rather than the value of the payload.... of course in a function node you can set node.status to anything so this can be abused fairly quickly :-0...
And yes it is fairly arbitrary. If it's something where you would "normally" expect it to be part of a flow eg http-request - then yes it makes sense to also handle the error as part of the output message. If it's something you have handed off to something else then status may be more appropriate. But somethings that fail a lot... do mean we change our minds... like the file output node now has an output.
Also of course if the node has an output button (like debug) then it shouldn't have a output pin as well.
