Yeah but Sleep 2 in obviously going to be on a timer since it's a JA [30+ for the Black magic one] And I doubt that's stepping on BLM's Toes too much. Especially if it's a sort of 1charge thing like Ele seal,etc.
Which means sure we can Sleep2, but on a [probably longer] timer, in comparison to the timer of BLM's Sleep2s. Not to mention if it IS [Which i'm guessing it is? Won't know till we get in there :D! ] one use per JA that if we use it for a different Black spell, we can't Sleep 2 if it's suddenly needed.
Unless they make it like a grimorie? .. I doubt they would though, that'd be so over powered.
So I don't think theres anything to worry about there with their ability to tap into higher teired black/white spells.
It wouldn't be nearly as reliable as bringing a BLM or WHM along for those specific needs in events,etc.
No worries yeah (ヾ('ω')?