Usually i prefer a black background to my images. But i also have a lot of older images, where i had those white frames (or partially even transparent) around all the pictures, which doesn't look so good with the black background.
The thing with "auto" is, that i would prefer the algorithm to decide only between two settings, either black, or white. The way auto acts now, is a bit too random in its color outcome. So maybe a "white frame" extra option would make sense.