The AI would sometimes use it. It does check whether it has several guns which one to use (afaik), but this would not work on armour. Additionally, a modder might want to explicitly have an enemy use a specific item (like, say, the leader of a criminal gang using a custom-set golden desert eagle). Without controlling the entire inventory, this might not happen (so on, say, higher progress values, the enemy might get spawned with a MG3 and use that, which is reasonable gamewise but not what the modder intended).

So only when you killed the cow did the enemies rise again ;)