oGCD oriented nature of BRD/MCH (IE as long as a BRD/MCH doesn't lose many oGCDs for the whole fight, you can look at LBing as losing raw BRD/MCH DPS without CDs, if that makes any sense), a distinct difference in DPS regardless between BLM and BRD/MCH, and BLM loses the most DPS period from LB3ing, it being particularly bursty in how it does its damage, and even if you keep Enochian you have to spend time after it ramping up.

It's not the most common opinion for BRD/MCH to LB3 over a BLM but I'm a firm believer in it, now and for future progression.