So I'm going to be the voice of dissent and tell you to fix your bumper. If it was more cosmetic, like a scratch or some paint rubbed off, I'd feel differently. But you're saying the plastic (or whatever material it is) has an 8 inch crack which, IMO, is something that should be fixed. Go get a few quotes. See how much of the $700 it would cost you to fix the bumper. It can't hurt.