I would definitely allow an NPC to use the Sacrifice Shield maneuver. As far as how common they are, yes; it depends on what kind of opponents you regularly face, but I use humanoids and undead more often than not, so about as often as you run into a PC shield user for me.I understand that can feel underwhelming for the player, I think the negative aspects are being a bit exaggerated:
First of all the possibility of sacrificing the shield is something that all PCs can do, but not necessarily NPCs can do. It makes sense that they can do so, but it's not guaranteed (npcs typically do not have death saves, etc, the game is designed to be asymmetric in many ways).
For monsters that do have a shield and sacrifice it, they lose part of the AC for the rest of the fight, which will result in higher DPR in all following attacks. In addition some of them may lose some actions, bonus actions or other traits related to shield use, which worsen their action economy.
Most monsters do not have shields though, so it also depends on how frequently they find enemies with shields