2 small 450mah batteries are very light and will last a day of flying. If for some unknown reason one side (battery or regulator) goes bad, you have a backup. It is insurance. I guess you get a little more tunability for balance point too. Maybe the cons would be extra cost, slightly extra weight (few grams for extra regulator and wiring?), and the requirement to monitor both sides to be sure to catch a malfunction before your redundant side gets drained.

It was suggested to me by the guys I trusted the most starting out to use a redundant setup. I went with it and it works for me.