Don't see a point in using them, the majority of monitor glass on cameras are gorilla glass which is almost unscratchable. If you damage gorilla glass then I don't believe an aftermarket bit of plastic or glass would help save the day.
I can tell you that the screen on my D3200 is most certainly not Gorilla Glass, nor any kind of glass. Alas, I discovered this by damaging it, in a manner that I wouldn't have been able to damage it if it was glass.
Having decided that I needed a screen protector on it, I improvised with one that was made for a Palm T|X. I put the protector on, and then used a sharp box-cutter to trim the edges of the protector. At some point, the blade slipped inward, cutting through the protector and into the screen itself.
I removed the damaged protector,and installed a fresh one, this time doing so very well, but as a result of this mishap, I now have a cut/scratch on the right side of my screen. Fortunately, it's not terribly prominent, but it's there. The blade that I was using would not have cut into any kind of glass, but it cuts very easily into any kind of plastic.
The dust that you see in this picture is actually in my scanner, that I used to take this picture. One of these days, I need to take my scanner apart and clean it out. I do not have any visible dust, bubbles, or other defects trapped between the screen protector and the screen; just that one cut/scratch indicated by the red arrow. It's a lot more visible in this image produced on my flatbed scanner than it is to look at it with my eyes. I can barely see it, if I try hard to specifically look for it, when the screen is on.