Absorb-Charisma would be useful when fighting bard enemies (yagudo are sometimes bards). You can absorb their charisma stat, which will in turn lower the effectiveness of the enemies songs.
Another use of this spell is helping a bard's song stick. if bard songs work like dark and light magic then CHA effects how well the mob will resist the song.
That makes no sense. It would only work if someone's job was DRK/BRD. You cant transfer the CHR you absorb from a mob to another BRD in your party, it goes to you.