I know these things have to be considered as part of the entire kit, but Ignore Pain just seems to be at least twice as good as Soul Cleave. It is an absorb, instead of a heal and has basically the same resource cost, but the heal value is double, especially after warriors get all of the various artifact and talent buffs to Ignore Pain. Soul Cleave does good damage, but even that is largely lost because Ignore Pain is off the GCD, so you can combo it with Shield Slam or Revenge. Or, if you don't need the absorb, you can do the dps and save the rage for later, which DH really can't do.
Ignore Pain - 20 to 60 rage- at 60 it prevents 90% of the next 2800% of AP, lasts 15 seconds.
Soul Cleave - At 60 pain does 742% AP as physical damage and heals you for 1200% of AP.
Maybe I'm missing something, lord knows I'm not good at the theory crafting.