[BoredAstronaut]

Single-item submenus and super-long submenus are to be avoided at all costs. They are not user friendly. The former has no reason for being -- put the contents in the parent menu as a sub-section. The latter is generally found in auto-generated lists, like lists of fonts or countries.

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.

[McP]

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.