Replying to What goes into choosing a park boot?

Im currently helping my brother buy a ski boot. He skis half park half all mountain. I ski the Dalbello il moros as they are great in the park and do a good job handling else, but recently I've seen a bunch of skiers who prefer high flex race boots for their boot and I was just wondering if there is some kind of logic behind this or is everything just preference? What technologies/features have a benefit in the park?