Trigger spawning really ruins the game, imo. It doesn't just ruin stealth (since the things that spawn are awake and mad at you immediately) but it also gets rid of strategy when clearing a poi. Like to pick guys off with a crossbow? Too bad. Worried about making noise in the POI? No problem, they don't exist to hear you. Want to go through the POI in a different direction? Well, you can be in a room that's clear but because you went in the "wrong" way, there aren't zombies and you'll have to go back later on if this is a clear.
I liked it when all the sleepers spawned at once and if you kited them too far from the POI they would respawn inside. If you didn't completely clear the building, they would all respawn when you left. If you made too much noise, you'd wake them up through the whole POI. If you were stealthing and you stepped on a pile of paper, you'd wake them up.