Especially at night. Here's Felix demonstrating at Curt Gowdy on his 83mm shell low-q fatbike in the daytime:
Now, I'm not convinced that fatbikes are a great choice for racing on dry trails like Felix is doing here unless things get *real* weird terrain-wise, but what I will say is that they are *awesome* for night riding. At night, you can still see big obstacles and the general direction/trend of the trail. What you can't see are the subtleties and the small obstacles - which a fatbike will happily ignore. I'm about 10% slower on my fatbike in the daylight going uphill, maybe 5% slower going downhill. At night, the uphill doesn't change much, but the fatbike is at least 10% faster - and 100% more fun (ie, I can let it rip and not be afraid I'll die because I didn't see a patch of sand).
Give it a whirl, folks! No reason to leave that fatbike hanging all summer, even if you've got other bikes to ride.