Add something like 'Use current desktop wallpaper' into 'Background:' dropdown menu in 'Theme' tab. For better coherence between system's elements in case for example "Bing's Picture of the Day" is used as wallpaper.
I heartily agree.
See bug 383670 comment #1 for an explanation. Plasma has many different types of wallpaper plugins, which would need to be supported by SDDM.