T13 adds

[MythToken]

Flash can be done before you can actually target the add. That is why its better, has nothing to do with potency. You can still do a loaded BB when it lands, or whatever you want. Personally I like Overpower in case he moves a bit.

This is just like T4, you can flash before the add is actually there and still hit it.

[Credor]

Flash can be done before you can actually target the add. That is why its better, has nothing to do with potency.

This is the clue. If add is non-targetable, then BLM cant steal aggro (Fire spells needs target to be casted).
Flash is played BEFORE, not INSTEAD. BEFORE add will landed and become targetable. Not when is targetable - this is too late for Flash, every dps can hit and tank must build strong aggro (like BB).
That early (non-targetable adds) Flash is often called pre-Flash.
So pre-Flash gives starting aggro to tank and 1-2s to hits with BB. I try first-hit-race many times without pre-Flash with 50/50 result (add stay on tank / goes to healer). With pre-Flash - add always stay, no matter what dpses doing. Even early autoattack (i.e. from BRD when swap target from Bahamut to add - AA counter is not reset after targets swap) will not start add's walking to healer, because pre-Flash gives aggro to tank. IMO playing without pre-Flash is risky: early auto-attack or race with dpses for first hit.