bump - Finally got the switch cluster yesterday and no wire diagram. Started mapping the switch positions to the wires.
As for usefullness, I'm not sure that I have ever used it to pass somebody, but learned to love the silly thing. We live in a wooded outer suburb to the north of DC. Rather than continually thumbing the big rocker, the flash switch is handy when you continually are switching hi-low. For example, if you want the high beam and know that the highs will only be on for a little bit before a car comes the other way. Coming to intersections with cross traffic, it's a great attention getter. etc.