Coming from road to MTB, I purchased a pair of knee pads for the same reasons listed above in this thread. I still wear them about half the time, but it depends on whether a given ride is more trail or more enduro.
I encourage my kids to wear knee pads (and sometimes elbow pads) most of the time, and they feel more confident with them on. This summer we bit the bullet and bought full-face helmets for both kids. partially because they are following their older friends’ examples and getting into some light downhilling, and partially because the local MTB summer camps typically require them. Personally, I don’t think I’ll ever ride something that requires a full-face helmet.