It's not narrow skis just for carving. Narrow skis are best for rotary turns also, because less swing weight makes it quicker. Basically, any time you want to turn quickly, a narrow ski is superior, whether it's edge to edge quickness, edge grip, or ease to rotate. This is why competition skis are in the 60s under foot. There are many design parameters. You can make a fat ski that grips, but it will never grip as well or turn as quick as a narrow race ski. You can make a fat ski smear, but a mogul ski smears very nicely without hooking, and swings around faster with less moment of inertia.
When you don't care about turning quickly, and you want more lift, a fat ski is better, or if you want more swing weight to bust through crud. Basically, once the powder is skied off and the slope forms into bumps, experts will rip the line better with narrow skis in the 60s.
Mogul skis smear very easily in slush. That's what they are made to do. They do it better than many skis even up to 100 under foot. The extra lift from fat skis is really only needed in deep powder. In all other conditions, the fat just slows down the turns. Good skiers are fine on narrow skis in most conditions. Giving up lift for quickness is a trade that many expert skiers are willing to make very often, though I realize that intermediates aren't as anxious to make that trade, because they haven't gotten to the point where they are working on fast turns.