The way the spell reads you only use up the blood if you draw the circle with it. Realistically, though, how often are you going to be casting that spell in a 24 hour period such that 1) your vial isn't hasn't hit its expiration date, and 2) the humanoid killed didn't have enough blood for 326153 small vials, so you only have 1?
Pretty regularly think. This pretty much eliminates the spell on the first combat after 24+ hours of downtime. Also on any adventuring days you spend scouring the earth of Undead, Dragons, Oozes, Fey etc. There are a lot of days you don't chop down a humanoid.
Play Tomb of Annihilation for example and there will be few days you do have the vial available.
The second consideration is that if you don't bind it within the circle, the odds are very good that it will soon escape and start eating peoples faces for 1d6 rounds.
The circle doesn't bind it, it protects you (and only you, not your allies). Although it is an hour long spell the save every turn means it is essentially a 1-battle spell, even if you know the true name.
Since you control the demon (until it makes its save), the idea is to cast it and then have him do what you want (which could be eat bad guys faces), and when he makes his save he keeps eating their faces for a few more rounds if you are lucky (or starts eating your allies faces if you are not lucky).
I have never actually used the spell before, but in terms of power, it appears to be the most powerful summon spell there is in terms of raw power. For a 4th level slot you can summon a Babau that gives you at will Dispel Magic at no action cost (as well as at will Fear at a low DC) or a Barlgura to rip faces off. A 5th level slot will give you a Vrock that has ranged stun, can fly and poison opponents. If they make their save, the go into beast mode and "spend turns persuing and attacking the nearest non-demons", which means with careful placement you can still make it so it probably won't hurt your allies.
Edit: I guess there is a use where you have it for an hour. Cast it in the middle of an enemy city and then teleport out of there but maintain concentration. The Demon will make his save and be uncontrolled before the hour runs out but he will go around rampaging until he is killed or you lose concentration. This is a highly situational use though.