Prior to 6.0, only BM could (as far as I know) get an MS (mortal strike) pet- the Devilsaur. That's still the best MS pet imo since he also still provides 5% crit.
But it's a pretty big deal that MM and SV can now get an MS pet, especially since a lot of the classes hunters look to play with (ret, dks, ferals) lack the MS debuff. There's a pretty huge difference between the enemy kill target having 25% reduced healing taken and not having that debuff. And since we no longer have widow venom, I think an MS pet is a no-brainer if your comp lacks the debuff otherwise.