Subject: Why does the nervous system of many metazoans 'cross over'?

I teach an Animal Physiology/Diversity Lab, and I'm asking because I don't know 
that there is a generally accepted reason for this phenomenon.  What, if any, 
selective pressures would make it advantageous to have the left hemisphere of 
the brain control certain actions of the right side of the body, and vice versa 
(e.g. optic chiasma)?  In other words, why (and WHEN, EVOLUTIONARILY) is this 
characteristic thought to have evolved?  Here is a scenario I think might 
confer the necessary selective advantage (unless this is due to neutral 
evolution (Kimura))--perhaps explaining part of the why portion of my question:

If the left or right side of an animal is threatened (e.g. by a predator) it 
would seem to be advantageous to have the side of the brain controlling 
the "threatened side" as far away from the threat as possible.  Are there any 
more well reasoned speculations or classical explanations for this "cross 
over"?  I can think of problems with this explanation, so I'm hoping that 
there's some additional insight you can provide me with. . .

Re: Why does the nervous system of many metazoans 'cross over'?