For most resort skiers, lightweight base layers (130–180 g/m²) are actually the sweet spot. They provide enough warmth for typical winter temps in the 20–35°F range while breathing well enough that you won't end up drenched in sweat during active skiing. If you ski aggressively, hike for turns, or naturally run hot, lightweight is almost always the right call.
The key is thinking about your whole layering system, not just the base layer. A lightweight piece paired with a decent mid-layer and insulated jacket will keep you comfortable well below 15°F. Where lightweight falls short is on those bone-chilling days when you're sitting still on long lift rides—if you run cold or ski mostly in single-digit temps, midweight is the safer bet.
Think of lightweight as your versatile, three-season workhorse. It's the most popular weight class for a reason: it handles the broadest range of conditions without overheating you.