For most recreational skiers, knee-only coverage is all you need. These pads focus protection right on the kneecap and the immediate area around it, which is the most common impact zone if you take a tumble on groomed runs or ski casually with family. They're also the lightest and lowest-profile option, so they fit easily under your ski pants without feeling bulky or restrictive.
You'd want to step up to more coverage if you're skiing moguls, racing gates, or charging big mountain lines. Mogul skiers benefit from shin coverage since the bumps can bang up the area below your knee, while racers need thigh protection to deflect gate strikes. But if you're not doing those things, extended coverage just adds bulk you won't benefit from.
The bottom line: if you're cruising resorts and just want peace of mind against occasional falls, knee-only pads are the right call. Save the extended coverage for the disciplines that actually demand it.