I agree it is just a myth probably stemming from the whole thinking that synthetics are the godsend of lubricants (egs. you can go 1 mil. miles with no engine wear, 25k OCI's, etc.).
Now that we know these myths are untrue, you can safely use synth's from day 1. However, why waste perfectly good oil. Keep what's in the car for 3,4-5k mi. and then start with the next interval...