Yes, I would definitely say that Sleep is useful to the game.
For FF10, I focused a little more on the characters who cast magic, so I would just put a creature to sleep and bombard it with magic spells.
It is better to kill monsters with the use of status ailments, because games are all about strategy. Plus, the bosses use them against you as well, so no guilt.

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May I say it was your fault
Because I know you could've done more.
Oh, you're so naive yet so
How could this be done
By such a smiling sweetheart?
My
Nook