It's really down to personal preference. Sometimes ball pythons- especially hatchlings, will only respond to the smell of a rat pinky, alive or F/T. Though in the long run, rats are better in nutrition compared to mice, I have switched the hatchlings (young balls) I can over to mice until they reach an appropriate size to down small rats or medium. A more developed rat has a lot more in it- calcium, iron, ect. Things that aren't abundant in pinkies. Is it bad to feed pinkies? No. I know people who have followed that method all their life with their reptiles. But me personally, I choose what has more nutrition at the current time.
I.E Adult mouse vs. Pinky rat. Adult mouse has more nutritional value than a pinkie rat.
When the snake can down a medium rat or even a small rat, you'll find the nutrition difference changes again and mostly in favor for the medium/small rat. Sorry, I didn't mean to say feeding pinkies will leave snakes undersized. But from my experience, all my animals grow faster on the adult mice diet for now. Again, all in perspective and preference.
-Ph.