Creighton,
When I came into the trade....... uh, musta been 3-4 years ago
the old geezers used to slap me up side the head for asking that question.
Sidewall bearing pressure aside, the "instruction" I received was that it had nothing to do with how hard we had to pull, it had to do with manufacturing a coil, and what that would do to our systems. I think it's probably a bit of both nowadays, but both do have their valid arguments.