I don't deny that they're useful. Freezing is very nice, especially for mêlée characters and fire wall sorceresses. However, I do state that they don't deal a lot of damage and are thus not very useful for getting a second damage type.

At level 99, a cold iron wolf has level 9 glacial spike, doing 73-81 damage to a group, and level 20 ice blast, doing 255 damage to a single target. Obviously, lower-level mercs have less skill points in each. They don't have cold mastery.

I think the sheer amount of damage is quite small compared to a mêlée merc with a good weapon. I think if an immunity made your character useless against a hell monster, the iron wolf would also take quite a while to kill it...