Monk's not wanting to play without heavy physical groups?

[Silver-Strider]

It's because 5% physical dmg increase is so negligible, the only thing to actually look at is what it does for you. With only one other physical attacker, you'll be lucky to even get a 5% dmg increase from Brotherhood.

That's the problem with RNG. You have to make the skill extremely strong, or else it feels underwhelming when you barely get one Forbidden off under Brotherhood.

I've gone whole fights with 3 Physical DPS/2 tanks and not gotten 1 Chakra from Brotherhood, the only thing that makes it worth it in that composition though is that at least half the party benefits from Brotherhood but, as you say, 5% is negligible anyways, just like AoE Balance or Embolden are negligible.