The old system led to such an imbalance of level gains and discouraged people from playing jobs that didn't level as fast as others. It also encouraged people to take their time fighting a single mob to get the most xp out of it rather then outright killing it and move on. A game that encourages you to level up through means of holding a mob isn't a good way to go about it, it's better to fight a horde of mobs rather then attacking something and waiting for the random number generator to grace you with xp gains.