if bungie wanted it to be a mandatory thing to deal with, they would have had the adds and the knights simply spawn X seconds after the ogre spawns. They did not do it this way, and thus it is a valid strategy. The knights still spawn, and still try to destroy the bombs (or you if your near it) we are just simply faster then they are and avoid the mechanic. Its no different then not killing the boomer knights on Crota to not have the wiz spawn. expect a change for challenge mode that will not allow this, but it wont be changed for heroic