Atm it’s a completely useless modifier.
Reasoning:
Every build playing BA is Soldier by default. Each and every competent physical Soldier build is going to max out Break Morale. Break Morale and the modifier on MGoUM don’t stack.
Things that would make sense: Stackable RR or a bit of flat Phys or Trauma to BA. Or a bit of love for the Spellscourge Variant, for example -10% phys res to IEE.
That’s actually something we’ve already done for the next patch.
Great news!
I just started leveling a spellscourge Battlemage and this is exactly what I wanted to see
how can you level with spellscourge set? 
To be more specific, hoping my damage will be enough when I hit 94 to bladearc my way to victory. So this was pleasant to see since I haven’t really seen much on spellscourge with the exception of “buffs please” or fluff’s build that doesn’t even use bladearc
When Fluff made that build, set wasn’t supporting Blade Arc . I think the set is ok now, considering absolutely lack of synergy in Battlemage class and again no -RR in either mastery.
The recent buffs to arcanist are why I decided to give it a go. I’m not a huge fan of war cry, it just doesn’t flow but that’s more of a personal preference. Adding some physical rr to the arcanist side would be neat though.