So, what is wrong with telling young vaulters to resist/bend the pole with the left arm to achieve a bigger bend and delay/slowdown the body from swinging trough too early?
It is wrong because
A. the bottom arm will only get in the way if this is a vaulter's concept of how it works and:
B. If everything is set up properly in the run and takeoff you cannot swing too early. The faster you swing the more the pole will "move" because of the pressure applied through the top arm.
However, if the bottom arm is not on the pole none of this will happen.
It's simple column physics, Roman. Energy applied down the length of a column will not collapse it until some force, however small, begins the process of bending it. I am not sure how you can maintain that a one armed plant will not turn the pole into a column as the force of the run-up is applied directly down the length of the pole. The top arm will be slowed because it is attached to the pole and with nothing to stabilize the lower body, the hips will be thrown forward until they line up with the pole.
The bamboo and steel pole vaulters had to combat this tendency because their poles did not bend, nor did they want them to. They kept themselves from being thrown past the pole with a precisely timed takeoff that moved the pole ahead of them so that their center of gravity stayed behind it.
I have talked with Dick Ganslen who was a 14' vaulter in the 40's during the bamboo era, and he assured me that they chose the stiffest bamboo poles they could find so that they would not bend. My coach at OU was J.D. Martin who was a 15' vaulter in the steel pole days. He told me that if one of those poles bent that they stayed bent, so he had to make sure his takeoff would not apply any force except down the length of the pole. In other words, I have two world class vaulters from both of those eras who have told me that, even though their poles could bend, they tried very hard not to bend them. This is one of the reasons why I cannot agree with your assertion that a takeoff identical to a stiff pole vault will bend the pole.