Single-item submenus inevitably occur at some point if the submenu is filled out programatically (as smackfu pointed out). A list of recently open files is a good example. While it's possible to move the single item into the parent menu, that would make the menu confusingly inconsistent.
Usually a menu is a bad UI device for selecting from very long lists. Combo boxes are one alternative. Breaking contents up into chunks and adding another level to menu hierarchy is another possible solution. MS-style hiding rare choices is a third option.