Since we will be playing in group, the only reason for Grave Injustice to stay is to recast Gargs more often through the rift. In solo it is quite useful to reset cooldowns of Piranhado and Wall of Death, so that potentially allows for faster stacking of Stricken on elites. But it's practically useless against RIft Guardian. If Rift Guardian summons no adds.. then we don't kill adds and we don't abuse GI at all..and we don't reset our cooldowns.
In group. Since we don't have Pirahnado and we actually stack Stricken FASTER because we spam haunt all the time.. So Grave Injustice becomes quite useless at stacking stricken against both Elites and RG. It's only there for slightly more often repositioning of Gargs..I doubt this is a valid reason for it to stay there and i believe Grueseome Feast or Pierce the Veil would be much better options. The %life that GI provides is nice, but again. We lose it all against RG fights if there's no adds.
GF will be Pierce without having to do any math or thinking. Barb and monk can run Solanium(minor globe increase) and barb runs threatening shout with the 15% to spawn globes(on AOE is easily summons 6+ per cast).
Looking at numbers 1 stack of GF is = to Pierce. Anymore and GF will always win(at 14k int, the more the faster it gets better and better).