Object based save handler does some hack.
It extends "previously registered" save handler. Previous save handler functions are registered as it's default save handler. i.e.

ini_set('session.save_handler', 'files);
$handler = new SessionHandler;

just creates files save handler if user didn't extend it. If user extend open(), it does not open file save handler and other method calls become invalid unless user extend them all. See relevant tests for details.