You are aware that only the initial damage is boosted by putting into a higher slot, right? Which makes it an even worse choice than using it in a 1st-level slot.
You're right! I didn't catch that. Yeah, it's a terrible spell.
If subsequent damage were also increased, it would be a pretty solid choice for warlocks (I'd say the name of the spell is a rather obvious hint) when he can think of nothing else to cast. Warlocks always use the highest spell slots available and can regain them after a short rest. Auto Xd12 damage per round could be pretty sweet.