I explained this in the other thread, but I'll explain it here again shortly and sweetly:

ANY amount of damage creep from RDM onto BLM will drastically reduce BLM's value. The ONLY reason to bring BLM into a group is its pure damage output, so if the gap between RDM and BLM is lowered, then the value of bringing a BLM is gone. So yes, in this sense, it REALLY IS zero-sum, whether we like it or not. (Why take BLM's marginally stronger damage if RDM does almost as much and buffs the whole party?)

Increased offensive utility is a must, but increased single target damage, on account of this, is a must not.