A second photographer is a great asset for several reasons. Primarily, if you like the reportage style, the second shooter is usually tasked with quietly circulating amongst the congregation during the group photographs. This is where natural images tend to be found. The primary supplier will be dealing with the groups while the second tends to go unnoticed and can catch relatives laughing or children playing. On a personal note, I've always found the long aisle shot of the back of the dress tends to be one of the bride's favourite images from the day. Once the primary photographer has taken up a position at the front, they can't move away. This is one of the many things that tend to be reliant on the second shooter. There's also the advantage of a second viewpoint that can't be underestimated. Not to mention the increased number of images available to the couple. The cost of an additional shooter is usually quite small when compared to the initial price, but the benefits are huge.