First of all you can postpone the damage done to you to 8 seconds in the future, perhaps to let a healer get out of CC or reset his dispel CD, or even get in LoS of you where you otherwise would've died.
Second of all you can reduce the damage component of Dark Bargain either through other CDs such as the 50% reduced damage CD, or you can absorb 38k of the postponed damage with the Shadow absorb. (I forget it's name off the top of my head) You can use dark bargain to effectively absorb 75k of any school of damage. That being said that's not a whole lot of damage once we hit 90, but I believe the absorption skill increases in value too, so it's going to be higher.
You can also double-dip with the 50% reduced damage CD (if you feel like blowing both at once) to get the 50% reduced on the initial damage, as well as 50% reduced on the 50% blowback damage, letting you buff it to effectively a 75% reduced damage CD.
All the talents in that tier have some negative effects on the warlock. Soul Link reduced your pet's total health by half, pact causes you or your pet to sacrifice 50% of their HP as a shield on you, and dark bargain lets you prevent all damage for 8 seconds and postpone 50% of that incoming damage to after, where you can further mitigate the damage yourself or with the help of your healer.