I wouldn't leave it on, personally. There's no telling what its reaction to heat will be; if the motherboard and/or battery gets warm you never know if it will soften or melt. In addition, since it's not intended for long-term use inside the phone, who know what will happen as it ages; will it turn brittle and flake off all over the inside, or turn to goo and drip all over everything?
Chances are that in the long run it wouldn't hurt anything, but I seriously doubt the tiny extra layer inside is going to be of any consequence in protecting the OLED from damage; after all, most of that damage is going to be coming from outside the phone and the film layer is all the way inside.
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