The whole point of potty training is to make a child learn that unlike whilst wearing a nappy, when in pants they mustn't go in them but should use the potty or toilet instead.
So what do we do, we go make a pull up, show the child they are like pants but actually it's still ok if they wee in it like a nappy - how does this help!?
I can understand using pull ups once the child is potty trained, say for bedwetting when the child knows the pull up is not the same as pants, but that's it.