Consider that the optional nano-texture screen is intended to reduce screen reflection and glare from external light sources. By contrast, the standard glass option already benefits from Apple's very effective and robust anti-reflection and hard-coat surface treatments.
Third-party testing of Apple's standard and nano-texture screen has demonstrated that the standard glass is considerably harder and more resistant to scratches and blemishes than the nano-texture finish. The nano-texture finish is much softer than regular glass - and as a consequence is more easily damaged.
Consider that the iPad screen is the single most expensive component of an iPad - whereas a good quality screen protector is a sacrificial layer that is easy and relatively cheap to replace.
Whilst the Apple Pencil has a soft tip that is intended to protect the screen from damage, basic material science tells us that a hard abrasive material (such as grit), will embed in the softer of two materials. Once embedded, the very hard material can be repeatedly dragged across the screen - and cause damage. This risk is obviously of greater concern for the already softer nano-texture finish.
A fingertip can easily “feel” grit an other abrasive contaminants - which you will instinctively not drag across the screen. When using a pencil, you cannot directly feel potentially damaging surface contamination.
As the uppermost glass/air interface is the optical surface is primarily the source of screen reflections, applying any kind of film or Screen Protector over Apple's nano-texture screen, while protecting the screen from damage, will entirely defeat the benefits of reduced screen reflectivity introduced by the nano-texture glass.
Also consider that applying any kind of adhesive product to the nano-texture screen can potentially damage the screen finish.
In summary...
The use of a screen protector is a matter of personal choice. Using a screen protector with the optional (and more expensive) nano-texture screen entirely defeats any benefit derived from nano-texture surface.
Now armed with an understanding of “how” a screen can be damaged - and the relative differences between standard and nano-texture glass and their relative benefits, you should now have sufficient knowledge to help you make an informed decision.