Comment by mumblemumble

As for why this is useful:

If objects are allowed to talk to friends of friends, that greatly increases the level of interdependency among objects, which, in turn, increases the number of ancillary changes you might need to make in order to ensure all the code talking to some object remains compatible with its interface.

More subtly, it's also a code smell that suggests that, regardless of the presence of objects and classes, the actual structure and behavior of the code is more procedural/imperative than object-oriented. Which may or may not be a big deal - the importance of adhering to a paradigm is a decision only you can make for yourself.