Technically, Petan did, Ehlers didn't but did lead in PPG. You are correct, it's not the be all end all. However, it certainly beats the prospect struggling. And neither Petan or Ehlers led their league as a rookie. Hopefully, indicators of future upside, but by no mean a guarantee of success.
I wouldn't exactly call those two busts either. Although I am a little concerned with Petan. While he looked decent the last 2 camps, I caught a Moose game a few weeks back and his speed looked average at best for the AHL level. At his size, you need to be quick to survive, never mind thrive, at the next level.