If nothing else it could be an interesting (and code heavy) challenge mode.
It could even be a very simple version: monsters have no non-hostile pathing and ONLY react to either the player's presence or noise. That would, at least, make it change the gameplay enough to warrant the challenge option.
Also, the monster's limited line of sight is there for a reason right?
The line of sight also brings the intresting question of cateye, considering it would get hell of a nerf here.
Monsters have a limited line of sight for the same reason that players have a limited line of sight: there isn't much of a tactical anything if all objects in a given room are visible the moment you step inside. As it stands, there's an initiative to abuse the line-of-sight conditions within the game's parameters, something which is pretty "meh" as a tactical excuse (boring for me, at least). Cateye, in the most open of areas, is practically a hack right now: tiptoeing until you see an enemy and killing it before it has the chance to fire, cover be damned. The fact that other monsters do not notice that this is going on when it's in their line of sight troubles me.
Having monsters react to noise, when such a mechanic does not exist in the game at all, is probably way too ambitious of an idea for me to suggest. Monsters reacting to fire DOES exist, and monsters reacting to monsters that react to fire would also be something I'd like to see.