Sch smn are the only poor sods that really have to change it often since you can't do both mind and int. Sucks to be them.

Blm occasionally drop a couple points in pie for mana thresholds for smoother fire rotation.

Tanks swap vit and str as they gear up in progression content to meet hp thresholds.(also swap around for pvp)

Poor raiders sometimes put stats in vit for hp thresholds if they can't afford melded accessories.

Some healers put vit for pvp and be really frigging hard to kill.

Those are the only good reasons I've ever heard or done.