Depends on the trails and consequences of crashing for me.
XC bike riding XC trails: Mostly just helmet & gloves. Occasionally kneepads.
Enduro Bike: Full face, kneepads, gloves.
Other than saving a few grams of weight. I don’t really see a reason to not wear kneepads, and a good half shell helmet with decent coverage. Sure it weighs a little more, but does it matter in a non-race scenario? Kneepads don’t really have to be uncomfortable, just push them down when climbing, and pull them up when descending!