Logged In: YES 
user_id=6380

It still looks like a potentially considerable slowdown
(since every directory added by a .pth file is scanned
recursively), and I'm doubting the usefulness.

Why should a site admin have the power to make Python search
in an arbitrary place in my home directory? That could cause
surprises too!

When you are regularly switching between multiple Python
interpreters, wouldn't it make more sense to have a few
aliases (or shell scripts, .bat files, etc.) that set
PYTHONPATH without exporting it? Methinks that many people
have dealt with this issue *without* feeling the need to
patch site.py.

(BTW I'm okay with the other changes, from your description.)