I see no need for it. Unless you tend to use your belongings hard or have long finger nails. Protective films tend to be reflective (not what you want for obvious reasons) or the antiglare sort which can make the display look dimmer/dull. Just don't abuse it and you should be fine.
I often have beach casting fishing rods passed right through the Jeep. They touch the rear window and are then suspended by bungy cords across the front headrests. It happened once in a Vauxhall Vectra that the rods moved (no one witnessed it and all denied it) and cracked the screen. (It's all on a different post if you search for it).
Point being that if there was a solid plastic screen that could be fitted around the screen to prevent accidental damage then I would welcome it.
Manufacturing and installation would have been peanuts so I can only presume that it was deemed uneccessary at time of manufacture. What a shame when reflected upon.
I also take on board that a plastic screen would no longer make it a touch screen. Damn the fancy gadgets