Standard Threads vs Weightless Threads

[yoda]

Recently I read Charming Python: Implementing Weightless Threads
(http://www-128.ibm.com/developerworks/linux/library/l-pythrd.html) by
David D.

I'm not an authority on threading architectures so I'd like to ask the
following:

1)What is the difference (in terms of performance, scalability,[insert
relevant metric here]) between microthreads and "system" threads?

2)If microthreads really are superior then why aren't they the standard
Python implementation (or at least within the standard library)? (where
my assumption is that they are not the standard implementation and are
not contained within the standard library).

ps. I fear that my questions and assumptions about threading may be
rather naive. If they are, it's because I haven't yet done any
significant research.