Ravenous Bugblatter Beast of Traal
To me this shows how weak sleep is. You're spending a limited slot resource and the opportunity cost of another spell, for Sleep which is only slightly better at dropping them then your at-will attack, may not affect them at all if your DM doesn't let you know the exact HPs remaining, the foe has an easier time getting them back up, and may not target whom you want it to (since it's lowest HP first).As much damage as sleep? Unlikely. Average is 14.5 if I hit with SA that is only if I hit. It creeps up to 18 if I have hex running. Average on a crit is 25, which is still less than sleep with a 2nd level slot and I would need a natural 20 for that.
Basically in the exact case where a foe has 1-6 more HPs then your attack would do it's more useful that at higher resource cost, and any other time I could use it (the vast majority unless your DM tells you exactly how many HPs the foe has) it is either inferior (an attack would kill without using a slot) or completely useless (foe has more HPs than the roll).