Are there any benefits to growing up with a sibling as opposed to being an only child?

Possibly. There may be some affect on social interactions and friendship. However, studies have been mixed and there is no conclusive evidence that having a sibling will improve social interaction.
Maybe companionship. As a father of 2 girls born 3 years apart, they are best friends and always have someone to play with, which is more important in our family than some, as we live in a rural area with few other kids nearby. But not all siblings get along so well. A good article that summarizes the myths and science related to only children can be found at http://en.Wikipedia.Org/wiki/only_child.