For pvp the DK adds a lot more than death grip. Assuming a frost/blood spec, in pvp you get:

1. Death grip
2. Chains of ice
3. 20% melee haste to group
4. 50% snare up all the time (frost fever)
5. Aoe stun (hungering cold)
6. Group heal (rune tap)
7. +10% attack power to group

and probably a lot more.