I realize there is a symmetry between the functionality of urllib.{quote,unquote} and urllib.{quote_plus,unquote_plus}, but why doesn't unquote function like unquote_plus and map "+" to " "?

The +/space mapping is unique to CGI query strings. The unquote function is used in a lot of places to decode parts of URLs (e.g. throughout urllib.py itself), e.g. the hostname or (more likely) the pathname can be encoded using %xx, and there a + does *not* mean a space. --Guido van Rossum (home page: http://www.pythonlabs.com/~guido/)