I carry 2 in my bronco, a 3.5# up front and a 8# in the cargo area. Some of the places I wheel require that you carry one. In case your vehicle, or someone else's, catches fire, they don't want the entire property going up in smoke.
What's the possibility/likely hood of an extinguisher freezing? I know they're under a lot of pressure so I don't even know if it's possible under non-arctic conditions.