Willem, that picture is really awesome - runes in lichen, who would have guessed? Where did you get the picture?
I definitely agree with blueheron and konscii that writing could be useful for intertribal communication, and i could see how the multiple spoken languages having one written form plays into that. I also really like the quipu idea, I once did a project on the inca and was amazed that they had no form of writing, and that became even more amazing as I was looking at re-wilding type stuff and the connection between writing and civ. THe signs thing also really makes sense, but where do we draw the line between literacy and drawing? Cave paintainings could have left a message, but were they writing? Are the symbols on the doors of bathrooms writing?
I also think that writing, maybe even printing could be really useful in the future in order to spread ideas and maintain diversity of ideas. Look at all the diversity and flowering of ideas that was made possible by the printing press.