Well, multiprocessing (the modules) is problematic because not everything can be pickled. If you are working with simple functions this can be fine, but often we use third party libraries that use lambdas (that can't be pickled). Often you don't know why something can't be pickled.

What STM (as I understand from blog posts from the pypy team) is that it provides real threading without having to worry about pickling.

And yes, it only helps if you have multithreaded programs. However, multithreaded and parallel programs are very relevant today.