i don't leave mine on the car all the time, just pull keep it in the trunk till I reach the garage.
Street parking on NYC = dings and scratches, this might reduce them a tiny bit but really, there's no avoiding it.
edit:
the one thing they do help with is when someone decides to ram into your car with a front mounted license plate holder that juts out, cause little imprints to appear in your bumper. this will prevent that.