My reasoning is that stuff like ``WEB_FIXTURE_HOST`` make sense to change them locally, so it is handy to put take them from the environment (Travis exports some env var instead of creating a ``phpunit.xml`` file for instance). On the other hand, ``driver_config_factory`` does not make snese to be changed in a local setup. It is a configuration of the testsuite itself, just because it is generic (PHPCR-api-tests goes even further by forcing testsuites to define a ``ImplementationLoader`` class and hardcoding it, but I prefer to make it configurableà).