Ah well, you may have a point regarding the pencil. I dont have one so have not considered a screen protector to enhance or improve the pencil experience. But, this reflects on Apple. Why is their screen and their pencil not giving users the desired experience? Why do many (some?) users feel that the pencil works better with a screen type which Apple does not see fit to provide?
I know a lot of people that don't have problems writing on glass - I, however, have never gotten used to it because I've never really taken the time to get used to it. My palm smudge from hours of writing bothered me so much I decided to look at screen protectors, which, in all honesty, haven't been perfect. On the 10.5 and Air 3, they made the pencil not as accurate.
The idea of a screen protector to me is: Less wear and tear on the underlying screen. Now, I know people use their iPads for 5+ years with no problems, but I'm a little nuts when it comes to that. This and, the ability to read next to a very bright summer window with no glare (my reading chair is right next to a massive window). But primarily, the Apple Pencil feels more like paper when writing on a matte screen protector. No fingerprints are nice!
The negatives? Not as sharp display. In sunlight, instead of reflection, I get a muted fog like effect. Eventually the hand grease builds up and needs wiping off - as I write, I can see what I wrote days ago on the screen. Swiping up on my iPhone and my iPad, I'll catch the edge of the screen protector and it feels horrible.
I don't blame Apple for this, I blame my "grass is greener on other side" mentality and inability to choose a path and stick with it. I'm always wanting to try new things.