Gary - There are many reasons to defer.
Games are won in the second half, so that is when you want possession. This is the main reason to defer, even under ideal conditions.
Also, when you defer, you get to select which goal to defend. Depending on the weather, the wind, the sun, the noisy fans in the end zone, this can be a huge consideration, especially late in the game when a field goal can often determine the winner.
On sunday, the weather may have been a factor. It can be a liability to possess the ball in cold weather, when receivers tend to drop a lot of passes. You may want your defense on the field more than your offense, and force the opposition into turnovers. So, you may win the coin toss, and force the opposition to possess the ball to begin the game. This is a similar tactic most teams use when going to overtime - its often advantagious to be on defense first, especially if you have a lot of confidence in your defense...