If we want to go for realism ( big LOL here =p ) wouldn't it be more realistic for weaker spells to be cast while running, and more difficult/powerful spells would root the mage? Like say fire, aero, stone, thunder, blizzard and eventually water I guess would be like smaller spells that could be cast when moving. Then the second ones may root you but have lower casting times than the third installment. Also maybe Cure could be used when running, but cura would root you with a lower casting time than curaga. etc.