Agreed. I would say that by using a module from a plugin, you're joining what "should" be two black boxes, and those black boxes should interact via their API, and if you ignore encapsulation, then here be dragons.

What happens if the internals of the plugin change, or the internals of the module? This would be the same issue regardless of the object model.

That said, I'm sure that there are many modules that do exactly this, the authors probably wanting to save the expense of a method call, but I think it is Alter that is in the right here, and the plugin in the wrong.

clint