Without the case. I love the cool Aluminium back laying on my naked body.
Screen replacement for the 12,9 is so expensive that it I wouldn’t dare to buy it without AC+.Other than phones I've never seen the need for a case. If you have pets or small children then screen protection (such as the keyboard) is probably a good idea. The screen replacement fee for a 12.9" iPad is not cheap.
You can use it naked but be aware of shrinkage when it gets cold.
Joking aside the back will get scratched up from sliding around on surfaces if you have no protection. Just from having it sit on the countertops will scratch it.
I use it with ASK and would have loved to have an Apple silicon case for the back, but don't have that option with the 2nd gen. I bump it against a wall or bookcase occasionally, but am overall overly cautious and therefore don't worry too much. I was worried about the camera, but my phone's camera has come into contact with stuff over time with no issues, so I feel okay with it.