From the way that you describe the high school technique you used, it seems like there would be a "hitch" in the crab step because you couldn't extend all the way.
drum corps started using "left foot always behind", so after a while that became "normal."
Did you march in a corps that did this? I would love to hear where you received your information on this one.
My thing is, I highly doubt that anyone in drum corps would subscribe to the "left foot always behind" method. On certain moves (uhm, moving right at a forward diagonal!) they wouldn't be able to extend as far, thus limiting their movement...or mashing their taters. Either way doesn't make for a good summer!