Nickie,
I was in the same boat as you. I would street park the car, and I would take "the El" to work, so I only drove it a few times a week. Things were all good until one night when some jerk in a Blazer scratched my rear bumper. He/She left a huge black mark from his/her bumper also, but I was able to remove it with bug and tar remover.
There is also the fact that my passenger side mirror was snapped off, but that is a whole different story but frustrating nevertheless.
Let me know what option you decide to go with in terms of trying to protect the bumper in the city. Another option I have heard is trailer hitch and leaving the ball mount after parking it and removing when driving so your car doesn't look as fugly.