I don't think anyone's advocating fully realistic (or should I say, painfully realistic) guard AI. At least I'm not. What I meant was a more situationally aware AI. An AI that could react to disappearing nearby (key word here, nearby, like in the same room) buddies by starting to look for him ("Hey, where did you go?"), not by outright raising an alarm immediately. Then you'd have to think ahead and be ready to take out the next guard before he finds the body or after searching awhile he doesn't find the body and becomes sure that his buddy is missing and will then radio it in (i.e. alarm raised).
Of course the game would have to be designed a bit differently with such AI in place, but it would not invalidate stealth gameplay. Quite contrary, I'd say it would enhance it. Wouldn't it be more rewarding to sneak by and knock out guards who are something else than simpletons with the most basic AI.