For completeness, the req.args attribute should be modifiable. Updating req.args is done in modules like mod_rewrite and so something that emulates that may find it useful. It also may be necessary when wishing to interact with mod_proxy where one wants to add additional query args to the request as it is passed through to the downstream web server.
One use within mod_python itself may be to allow GET style query args when a POST request has been submitted to be wiped out before util.FieldStorage is used to process form fields. This would avoid using the hack approach of creating a wrapper class around the request object which returns an empty string when req.args is accessed.