I once made the mistake of placing too much faith in the healing abilities of the Statues by going CL/DL/Lich. Very powerful crowd control, but one problem: neither the DL or the Lich are capable of surviving focus fire without a DK. Something that slipped my mind, but did not slip my enemy's. I lost the DL and the Lich an embarrassing 6 times total, because no matter how much I danced away, the focus fire was just too much.
Despite the formidable army healing abilities of the Statues, keep in mind that army units rarely take focus fire. Heroes, on the other hand, do. That's where the DK steps in. The DK is capable of negating that focus fire quite rapidly through the use of Coil. 'Nuff said.
Even if you didn't need the healing, the DK can still boost the speed of your army, and can be employed as a powerful sniper. At 300 damage, he can hit any target harder with a single attack than any of the other Undead heroes can. In fact, the only one with a sniping attack of that caliber is the MK with a level 3 Bolt, and the DK is still capable of hurling the Coil more times in a single sitting (Coil's 5 second cooldown vs Bolt's 8 second cooldown), and the DK's mana is better able to handle it, especially with the Statues' ability to heal mana.