I'm not sure why those who created the B&C system included a measure of symmetry, but symmetry is thought to be a sign of health:
http://www.nature.com/news/1998/9808...s980806-7.html

http://www.livescience.com/4002-symm...uman-bias.html

So if the point is to measure the actual [I]quality[I] of the animal (as opposed to its appeal to humans), it makes sense to me to include symmetry in the equation.