Why would one ever chose just the running lights?
Well, fair question... how about because you can (or should be able to)

Sometimes it's useful/cool to use parking lights in conjunction with fog lights, and of course, there is the possibility of actually wanting to park the thing while leaving some indicator on to prevent people from running into it but not wanting to leave the headlights on.
Also, I'm toying with the idea of mounting Lucas fender-marker lights on the mudguards and to have them on as running lights and also function as turn indicators, but I don't think I want the headlights on all the time.