What's the idea behind adding a sidebar to Desktop Settings? It has only one category anyway. I think it looked neater on KDE 4.2...

My guess is that maybe the things that are in System Settings -> Desktop will go there in the future. Or maybe not..just a guess

By Desktop Settings I guess you mean from Plasma?
If so, it is likely because some modules offer pages to perform their configuration, however it is not leaving KPageWidget in "Automatic" mode, which allows it to determine when to use a tree or icon "sidebar" or not show one at all ( this is what System Settings does.. )