Persistence of contralateral innervation

[David Franklin]

Why is contralateral innervation so prominent and conserved a feature of 
the vertebrate nervous system? What is the functionality of this feature 
in the higher vertebrates? How does one explain the exceptions to this 
general rule, e.g. bilateral innervation of the upper half of the face in 
man (contrasted with largely unilateral innervation of the lower face)?