I completely agree with the OP.
Nobody uses Skull Sunder on MNK for any serious activity. Or Savage Blade on a Warrior, or Haymaker on DRG.

Classes have their attack skills and combos. Nobody would waste a combo for a weaker attack, no DD tanks a dungeon or raid, no WHM uses Blizzard over Holy.
Cross Class skills should either open new ways to play a class or supplement its existing skills.


The cross class skill system is something they adapted from FFXI, where classes had fewer skills with longer cooldowns and where you had to give up all abilities from one support-job if you wanted something from another support-job.