Games have a long history of pairing the player character with a companion, to vastly varying reactions. There is a long history of much maligned “escort quests” where the fail state of a mission, quest, or level is the death of an AI companion that the player has to work around trying to keep alive. In many RPGs companions are a part of the customization of the game, in Skyrim or Fallout, the companion you choose to accompany you, and the way that you interact with them is an essential part of the game. Sometimes AI companions can be entertaining, sometimes annoying, and occasionally, endearing. In combat heavy games, such as Bioshock Infinite and the new God of War, the only way to make these characters endearing rather than annoying was to make them immortal. Neither Kratos’s son nor Booker’s ward, Emily, can be injured in normal, unscripted combat. Although of course this makes the games slightly less realistic, it also helps to preserve the emotional bond between the player and these AI controlled characters. Inevitably, if they were allowed to take part in combat, they would occasionally be killed and either need reviving, which feels like an arbitrary punishment when you had little part in their death, or they would take actions against your wishes, frustrating your ability to plan out combat in predictable ways.
I think the fundamental difficulty with these AI is that, regardless of their stats in combat, they are unable to communicate with the player or plan ahead, and in modern times, still get stuck on geometry frighteningly often. If AIs are made that improve on these flaws, able to communicate, plan, and walk around rocks, perhaps the crutch of immortal main characters will be able to retire, while still allowing for interesting characters.