A DK without spell pen is making a big mistake. Deathgrip is the most clutch spell, and if its resisted , you just missed an important opportunity to grab the kill target. Since they can resist it 25% of the time, you would be a fool not to get spell pen.

It might be less necessary on a 4 DK team when you have access to multiple pulls, but its still only 4 gems to get 200.