Invincible AI is good, at least with current AI technology.
If you were to make current AI mortal it would be extremely frustrating, because quite frankly, no one has programmed good friendly AI yet; although that's probably deliberate, as I'll explain in a minute. They're likely to get themselves shot, or run in front of your gun and generally screw you over; the AI in the Call of Duty franchise wouldn't last 5 minutes. Having to revive your NPC teammates all the time because they're dumb will swiftly become a chore. Of course, the solution to that is to make the player not punished for the AI's cockups; if a NPC dies, then the player can carry on regardless.
Except that means you can't have plot-important characters anymore, since you can't program a game to change its story depending on where they die, as there are potentially infinite points in the game they could die or not die. So everyone just turns into a Red Shirt, contributing nothing to the game's narrative whatsoever. Of course there are plenty of games where the NPCs are mortal and their deaths can change what happens, but that only works in a restrictive RPG-side quest way.
You could just pull a Halo 1 (i.e. that mission where you have to escort Keyes out of a Covenant Ship) and make it so if a critical NPC dies, you just get an instant Game Over...and then we get escort missions which aren't fun because of reason 1. This could be avoided by simply making the AI very competent, but then that almost completely removes the players involvement as they can sit there whilst everyone else deals with the threat. This is why I think friendly AI is deliberately incompetent - especially since it's entirely possible to reprogram some AI such as the Left 4 Dead bots with some simple console commands.
You can make it so the AI bots in Left 4 Dead stick to you like glue and react insanely quickly to enemies; the default values for how far away the player has to be before they'll move is deliberately programmed to be higher, so that the Player actually has a possibilty of failing. Hypothetically, you could program an AI that's just the right level of competence - they'll hold their own, fight for themselves and not constantly go down but they will not succeed without the player's help, but I think there needs to be substantially better AI technology before that can happen.
Tl;dr:
Mortal dumb bots = frustrating
Mortal clever bots = makes player irrelevant
Mortal dumb but insignificant bots = no plot development
Making NPCs immortal is a way to solve the first problem without causing the other two.
