I apologize for the brevity - after all it was an "oldish" post - and I clarify after reading the useful link, stabilizer hitches are not allowed at my latitudes. For these it makes sense not to grease, since they work in a particular way, similar to my bike-carrier (Italian-made), and also my junk-carrier (UK made) do not recomment a greased ball for the same reason, and in that case, I tend to wipe clean so the clamps have greater grip on the ball.
But for all other purposes, both for my small >750kg and the larger car-hauling twin axle, I do use grease.
Just on the basis of protection against wear and rust is sufficient. These are expensive things to replace, and extremely dangerous if they were to fail.
Yes, greasing stuff might sound antiquated, but it is part of the normal prevention of friction wear between metals.
With me, at the question why they don' do it, most answer they do not want to have dirty pants ....
About the "hole", I was thinking to make a plastic cover with some velcro, it is enough out of sight so it would not be ugly, and it can be easily popped out when needed. While would not be a totally enclosed result, I guess would be enough to minimize dirt ingress in the tow-hole (lke the way this sounds!).
I tend to regularly spray everything that gets in the way of the elements ... I guess I am also antiquated myself 😜