Sure you might be able to write him off a little in game, but the thought that Harbinger was controlling a collector general who was controlling his troops not to mention doing this from god knows how far away is a little scary to think about. Think of how powerful a being such as that must be. Also, "You only damage the vessel, you cannot hurt me." Just knowing that no matter how many times I killed him he was just going to pop back up and continue to dislodge me from my bloody cover made him a formidable adversary.