I don't feel that it makes sense to use film/plastic to protect glass. If its something that could actually hurt the glass screen, then its probably going to railroad the plastic.
It reminds me of a story. When I was at the eyeglass store with one of my kids, the sales guy tried talking me into a $150 pair of sunglasses. I told him that I go through 2 or 3 pairs a year because I always lose or destroy my sunglasses and I would never spend more than $20 on a pair. He told me, that I wouldn't hurt or lose them BECAUSE they were expensive and because I would enjoy them more. I laughed.. but that was almost five years ago and I still have those $150 sunglasses and they are still in pristine condition.
Moral of the story is: Don't put things of value in harms way.. ie with Diamond rings, sand paper etc
or get a full case to protect it.