I really don't see why you'd want to unless you'll use the fat tires at least part of the time. The frame design has concessions made due to the fat tires, like the 100mm wide bottom bracket, offset frame, etc. that could be avoided with a decent touring bike or something like a Salsa Fargo.
I'm looking at fatbikes right now, and they're so much fun to ride that I don't see why you'd want to run "skinny" 45's on one. They're like a pedal-powered monster truck and make you grin like a little kid when you're on a ride.