This is bad for many reasons, namely you have no way of instantiating a Foo without using a file, in this case. There is nothing wrong with adding functions for read/write, as this is common in serialization idioms (they'll even call other methods of child objects, etc).

As the more versed in softare design will know, using an existing module has nothing to do with OO, nor does OO have anything to do with reusability. It is commonly parroted that non-OO code is not reusable, and it may better be said that non-namespaced code is not reusable. It has nothing to do with object-orientation. Structure-oriented code can easily handle reuse, OO when yielded properly has greater advantages of course -- such as polymorphism.