When I was widening my front track, and I removed the rubber strips to clean and then recondition them with 303....I accidently learned that the front track didn't actually need the rubber strip in place...the window went up and down just fine, and wasn't scored or scratched.

I did it over and over, and it was fine...but since my rubber was in good shape, I reinstalled it anyway as it just seemed wrong to leave the glass edge unprotected in the metal channel.
I then always kept the rubber track well lubed with pure silicon spray or teflon liquid.