That is technically illegal in many states. Parking lights are exactly that, for parking. In many state's you cannot legally drive with your parking lights only on. Either full on or full off. That said, I have never seen anyone get pulled over for it. Parking lights are quite possibly the single most useless feature installed in every vehicle. In 15 years of driving I have never once found a real need for them. Parking lights used to start out as a separate circuit with an independent battery, in which you would activate them when the car was parked out in the dark. But now they are tied in with the rest of the electrical system and no one uses them, dark or not, they just park their car and leave it. Thus the parking light system has become a bit of a vestigiality organ where every car has it, but no one actually uses or needs it.
Interesting notes on history and illegality. However, I quite disagree about being useless. One of my pet peeves while driving relates to all the abusive headlights - on when not needed, misadjusted, high beams in you eyes (towards you and from behind!) - one spends entirely too much time moving mirrors, closing one eye, moving your head, all to get some idiot headlights out of your eyes so you can see the road. Driving with "parking lights" only (when headlights add nothing to the driver's ability to see) is a great way to improve everyone else's ability to see you without causing them any problems. We just rename "parking" lights to LLRLs (Low Light Running Lights) and outlaw DRLs! What good are DRLs from behind anyway?