I just saved one sim from Grim after she collapsed from freezing. She was on the street in front of my sim's house and my sim convinced her to group up. It would take too long to walk back to my sim...
Individual pieces of clothing should be coded with "hot", "cold", or nothing—and treated that way, regardless of outfit category. That way, the hot and cold categories still work, but played Sims could also, say, have a winter wedding if they wore cold-coded clothing in the formalwear category.