Erlang – Concurrent Language for Concurrent World by Zvi Avraham.
If you need to forward a “why Erlang” to a programmer, this set of slides should be near the top of your list.
It includes this quote from Joe Armstrong:
The world is concurrent… I could not drive the car, if I did not understand concurrency…”
Which makes me wonder: Do all the drivers I have seen in Atlanta understand concurrency?
That would really surprise me. 
The point should be that systems should be concurrent by their very nature, like the world around us.
Users should object when systems exhibit sequential behavior.