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The reason for this patch given in the referred-to post is:

"We have been told that this directory is inappropriate for vendor 
supplied packages, just as "site_perl" is inappropriate for Perl. 
With Perl, vendor supplied packages go under "vendor_perl". "

where 'this directory' is site-packages, which works fine.

The python-dev thread subequent to this posting starts with
http://mail.python.org/pipermail/python-dev/2005-
September/056682.html
A subsequent post
http://mail.python.org/pipermail/python-dev/2005-
September/056696.html
clarifies that 'vendor supplied packages' here means packages 
installed by the system/OS vendor.

Disconnected (in the pipermail archives) pieces of the thread 
start here
http://mail.python.org/pipermail/python-dev/2005-
September/056697.html
and here
http://mail.python.org/pipermail/python-dev/2005-
September/056702.html

This last suggests that this proposal is on hold while a .pth 
solution is explored.