Hoobley, you are an encyclopedia - better even. Amazing the detail you seem to know about pretty much everything!
I'm no expert, but I wuld think that the ob is concerened that if the perineum is "short" that any tear, even a tiny one, would be severe... just my interpretation.
To be honest, it sounds very strange to me. But obviously a few OBs and midwives have come to the same conclusion independently.

Saša