You bring up a good point. I believe that the best thing to do would be have the lights turn on any time the windshield wipers are on.
Last September I was riding with a business associate on a road near Eindhoven that was like you described. The road was like a tunnel through the trees which met each other above the road. This is a different situation that would, of course, warrant headlights being on.
The auto setting on the newer MB's headlight switch will fix this. The lights will come on in subdued light.
Have a great day,