I have one thing to point out and then I done arguing my point. I run a dedicated set of winter tires, as does 70% of the population up here. It's just what we do. I'm not going to get into cultural difference, I already went there in a "do you drive with a gun" thread which didn't turn out as planned. Its your rig and your life, do as you please.
As for the "I drive in 30inches of snow in my summer tires with no problem" comments. Congratulations on your large penis, but guess what? Driving in large accumulations of snow is about as predictable as driving on pavement. Ice is the killer, I'm taking about when you only see the stop sign at the last minute (we've all done it) and have to slam on the brakes or when your going 60mph on a dry paved highway but them you come up to a bridge that is a sheet of ice. Thats when you'll want winter tires.
Just my theory...But as MtnBkr (who turned out to not be hard headed, I retract my comment

) said, maybe the severity of Sun Valley winters doesn't warrant them. I don't know I've never been there. I'm just giving my opinion as I'm a "person from a cold climate"