Karl wrote: I think the theory is that whichever electrode
has the least resistance will
receive the spark ...

Another theory might be related to the fact old American
iron hot rodders (and I'm sure others) would "index" plugs,
whereby they'd use different thickness washers to ensure the
open end of the ground electrode would face the center of
the combustion chamber, supposedly maximizing success of
combustion.  It's also known that mixture properties vary by
location as the fuel/air mixture enters the combustion
chamber.  Three-electrode plugs should address these issues
nicely.

For related info see
http://www.ca.sandia.gov/CRF/index_4.html