Don't listen to anyone that tells you leveling is good practice. Almost every job in this game plays differently at level 80 then when you are leveling. You can practice once you hit 80 and that would be far more productive than trying to learn the class as you level. That being said, whether it's worth it to buy a level jump is up to you. If you feel like you are wasting time in low levels and want to skip to higher level, then do it. If you enjoy the grind then keep leveling.