PEP 370 question

[Paul Melis]

In (the recently accepted) PEP 370 it says

"Current Python versions don't have a unified way to install packages 
into the home directory of a user (except for Mac Framework builds). 
Users are either forced to ask the system administrator to install or 
update a package for them or to use one of the many workarounds like 
Virtual Python [1], Working Env [2] or Virtual Env [3].

[...]

The feature can't be implemented using the environment variable 
PYTHONPATH. The env var just inserts a new directory to the beginning of 
sys.path but it doesn't parse the pth files in the directory. A full 
blown site-packages path is required for several applications and Python 
eggs."

I'm confused. For years I've been installing packages and extension 
modules locally using distutils's --prefix option, e.g. using

   python setup.py install --prefix=$HOME/local

Together with adding $HOME/local/lib/python2.4/site-package to my 
PYTHONPATH this works fine, even if there are packages that require a 
.pth file to work or that use Eggs (like setuptools and pygments).

So what limitation does the quoted paragraph in the PEP refer to?

Regards,
Paul