The problem with sleep is, that it becomes meaningless, after you learn fire 2.

Its nice for early game to have cc, because in the story fights would the enemie make to much dmg, if you dont take them 1 at 1.
But, with aoe attack becomes it much faster to simply attack them at once, instant of only 1 at time (not sure, if there are later fights, where sleep is better, because i did rarely solo story fights with blm -it looks very often to hard, with no healing power-).

The only way, sleep could become usefull again would be, to have solo fights, where its needed more to take the enemies 1 at 1 as all at ones with aoe.
Or, remove the part, that sleep is instantly canceled, if the enemie is hit (and make it practicly the mages stun skill). Butbthat would mean, that the duration would probably be shorter.