Yes it does avoid many problems, but it's unsuitable for other important scenarios. We need both. Message passing should be a design choice, not an excuse for a limitation in some language runtime.

That's interesting, which scenarios do you consider it unsuitable for?

The scenario I deal with most of the time is a large, read-mostly, in memory custom data structure that is accessed by multiple clients. More and more data fits into memory nowadays so that's going to be a very common scenario for many data analysis tasks in the future.

You mean CSP-style, like Alef and Limbo from Bell Labs? Have you come across PyCSP? It has multiple backends, including greenlets which are nice and lightweight. http://code.google.com/p/pycsp/

Stackless is inspired in that paradigm too.