I have a flow that just stores MQTT messages in a log without looking at their contents at all.
I could set the output to "a Buffer" and it would work. However, this also makes the message hard to read in debug nodes, so the "string and buffer" mode is perfect for this. Strings can be stored in the log just like buffers.
The "parsed JSON object, string or buffer" mode adds additional complication: I have to re-stringify the messages before I can put them in the log.
I know, I'm using it very successfully, hence the feature request to un-deprecate it. It is very useful so it would be great if it would continue to work in a future version of Node-RED.
Must admit I'm not aware of any current intention to remove this (@Steve-Mcl ?) - so yes I have no objection to leaving it there and removing the comment instead.