I agree with you, alot of Singleton classes are mearly modules shoe-horned into classes. But i disagree that this is reason not to use OO and instead use a plain ole module. It is largely dependent upon the context. While I always advocate mixed paradigm programming within the larger application, I think that on a smaller scale, paradigms should be reasonably seperated.
If my application is largely OO, then a Singleton approach/syntax makes sense. A Singleton's singular instance can be passed around and treated like any other object, the other objects blind to its Singleton-ness. In this case the module approach/syntax would be awkward and clumsy.
Now if my application is largely procedural/imperitive/functional style, then the module approach/syntax makes much more sense. It fits the context here. Where an Singleton object would be akward and clumsy.
Ideally a module should provide both interfaces (if appropriate), much like CGI.pm. (yes i know CGI.pm just auto-creates an object instance for you if you use the non-OO interface, but hey, encapsulation isn't just for OO).