Predictable spawns are fine. Exploitable spawns are not. You should be able to predict where they're coming from, but you shouldn't be able to shoot them right when they spawn or assassinate them right when they spawn. Safe but predictable is the key. However, I also think in Slayer that you shouldn't be only allowed to spawn on one side of the map, because you don't have an objective to defend or attack, so there's no point in limiting the safe spots you can spawn from. And despite what people would like to say, non-sided spawning in Slayer is not random.
Assassinate when they spawn of course shouldn't happen. Shooting when they spawn though, I'm ok with because I know it depends on the map. The spawning on one side of the map is also on a map by map basis. For example on Zealot Slayer, I think spawning anywhere works. The map is a little too small to not allow it. However on a map like Sanctuary, I think it should be one sided. I remember when Bungie changed the spawns on Narrows TS from one sided to spawn anywhere and it ruined the map.