The question wasn't about dropping it on enemies. The player who questioned it's use was asking about whether you could drop it out of sight of the caster with the teleporting mechanic. That player feels it's too strong but I can't see anything that says the caster can't move it to a location that is out of sight providing they feel legitimately that they could drop it in that location.
The situation has come up twice where players really questioned the mechanic.
In encounter A, they were fighting baddies in a room with corridors going off to the sides which hadn't been looked into because the combat started. Guards were heard coming from one tunnel. The caster moved the Cloud of Daggers down the corridor (which was 10' wide so the spell could go there) in an attempt to block them off or at least slow them down. I don't have an issue with this one because although they don't know the length of the corridor, they do assume from the entrance that it was wide enough to put the spell.
Encounter B was a little after encounter A, and the party were at a door where they could hear canine barking somewhere beyond. The Cloud was already in effect so the caster still decided to move it to the other side of the door to block it off. By my map there was no enemies at the door and it was clear space. The querying player was asking whether you could drop the cloud behind the door since you don't know what was directly there. Again, I don't have an issue but the player was politely unhappy with this ruling. Hence my reasoning for asking and checking with more experienced minds.