Well, without trying to drag this too far off topic, they do get 2 attacks each, a much better save than TK chariots, and cause fear. The important part about causing fear is they are also immune to fear, so they can be used to confidently flank undead, which a lot of other things in the LM army cannot.
With a unit of 5 cold one knights hitting the flank of an opponent unit, there is very little that will survive. As flankers you can keep them hidden by terrain and bring them out when weapons are either destroyed or have LoS blocked by combats.
I guess it also does depend a lot on the rest of your army too.