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I am not sure that this guy's idea is good, but I think that he deserves
more attention. In comments elsewhere on the web, I noticed that people
thought the PYTHONUSERBASE site-packages directory could contain .pth
files which would serve the same purpose as his proposed environment
variable. However, .pth files are not recursive; a .pth file in one
directory does not cause .pth files to be processed in the directories
named by its contents.
I agree with ianb that this is a poor way to mimic virtualenv. There is
a difference between user additions -- which should affect *all* python
code that he uses -- and separate Python installations.
There might be other reasons for multiple user site-packages
directories. For example, Python lacks Perl's architecture awareness. 
Perl can load from
 lib/
 lib/5.10.0/
 lib/5.10.0/linux-x86
 lib/5.10.0/linux-x86/auto
and a few other combinations.
Python also lacks a PERL5OPT equivalent. I am forced to use the
PYTHONUSERBASE mechanism to simulate that behavior (which I use to turn
on code coverage everywhere during testing) but I only get to set that once.