Same here. Very annoying for those of us who like to keep clean folders. I don't mind that it exists somewhere, but the top-level of my Documents folder is not the place for it. How about one of those Adobe cache or preference folders instead.
After latest Update to PS Version 2014.2.2, the DesignLibrary_Photoshop.log still appears under my Documents folder. Can I manually delete the file or should I wait for the next update?