Knee pads with lower thigh coverage are specifically designed for skiers who need protection above the kneecap — and that mostly means race skiers. When you're running gates, the plastic poles routinely strike the leg just above the knee, and that area can take a real beating over a full run or training day. The extended coverage shields that vulnerable zone from bruising and cuts.
For most recreational and all-mountain skiers, thigh coverage is overkill. You're not likely to take repeated hits to that area, and the extra material adds bulk that can make the pads less comfortable under your ski pants. A simpler knee-only or knee-plus-shin pad will serve you better for everyday skiing.
If you do race or are training in a course, don't skip the thigh coverage — it's one of those things you'll appreciate the first time you clip a gate at speed. Just know that these pads are slightly larger and may require a looser-fitting pant to accommodate them comfortably.