2 tends to be very valuable, too
when you get missed PATs that then means teams have to go for 2 and if they miss....
also - if you have the dog +2 and the game goes to OT you are just about assured a winner
whoever gets the ball first and scores a TD goes for 1 - and then the other team if they score goes for 2 figuring "we'd rather have the game's fate in our hands on offense" so +2 is valuable because of the way OT is