I can think of absolutely no reason why BMW did not provide a defeat switch for the auto dimming feature.
There are some driving conditions where I find that the dark mirrors make it difficult to see what is behind me such as when driving on crowded freeways after dark. There are some situations when I want to be able to reduce the glare from bright lights behind me and I want the mirrors to dim.
Drivers Club at Lime Rock
The Glen Club (Watkins Glen International)
International Motor Racing Research Center
Cayman Club Nor'Easters
Porsche Club of America
Madison Ave. Sports Car Driving and Chowder Society (Only a Vice President)
Sports Car Club of America
Polish Racing Drivers of America (PRDA)
American Mural Project
Simeone Foundation Automotive Museum
Mount Washington Observatory
Society of Automotive Historians