I think I've come up with a good way to make the Sleep status actually useful. What if inflicting Sleep on a monster first removed any current status effect on it, and then inflicted a sub-effect even if the monster wakes up early, that makes the monster immune to status changes of any sort (excluding more sleep and death, if you consider death a status change) for an amount of time based on the strength of the sleep effect? (something like 2 seconds, then 4, then 6, maybe?)

Sleep would no longer just be a watered-down crowd control that doesn't affect constructs and heals monsters when it wears off instead of hurting them like freeze. INSTEAD, it would be the nullifier, and also the safeguard to conditions you really don't want a monster to BE in.

Sure, we already have Freeze to cancel out fire, and vice versa. But Sleep would work on either equally well, and would also pacify ricocheting quicksilvers, so you could carry a sleep vial (or maybe some day a sleep weapon) and be ready for any of them. Better yet, the preventative effect would mean that if you extinguish a flaming oiler with sleep instead of freeze, you'll have a couple seconds in which you can try to kill it before any other, still-flaming oilers can re-ignite it again. Same goes with the quicksilvers.

In addition to regular status effects, sleep could also remove Attack Up status such as Alpha wolvers grant to their smaller kin, or the Trojans grant to themselves. Perhaps sleep could even delay the explosion of a blast cube in some way.

While it is true that an uncoordinated team might suffer frustration if somebody tosses a sleep vial immediately after someone else went to the trouble of lighting an enemy on fire, (or worse, cursing an enemy) that sort of thing already happens if say, someone uses freezing weapons after someone uses fire, or... anyone does anthing to a frozen monster. The limited number of vials one can carry would also limit the extent to which someone could accidentally or intentionally grief the status inflicting members of the party, anyways.

I feel that if sleep worked this way, a player MAY even give consideration to a sleep-inflicting weapon, if one were to be added to the game. While it is true they'd be missing out on weapons that cause more damaging status effects, they WOULD however have a weapon that could make certain monsters much easier to fight.