I think Singletons are ok when the core language's runtime promises to take care of them. They are also ok for things like the abstract state objects in a state machine and for special immutable Sentinal objects (e.g. Lisp nil, Python None, ...).
Good point: things like states, true, false, and nil make sense. I think the commonality there is that those are all immutable values whose only purpose is to have a fast object identity comparison. In a way, they're similar to global variables which are all members of the same class, except that it's nice for them to have separate types.
In fact, you could think of Python's None as being a singleton, but True and False being the two allowed instances of a, uh, doubleton? And similarly, states might be implemented as a limited set of allowed values of a single class. This reminds me of Haskell's algebraic types.
This maybe isn't quite "Singleton", but it is at least a useful case for limiting the number of allowed values of a class.
