Only problem is that it only applies to tournament-legal BFCs, so the ignore effect on older cards (such as Disbanded) are still a mystery. Or at least unclarified.

Suffice it to say that theme-ignore is NOT to be done in a knee-jerk fashion.

You are correct. For example:

-both teams are comprised of Batman Ally figures.
-your team is a keyword theme team. The opponent's is not.
-you field Disbanded! The opponent fields Ordinary Day.
-you ignore Ordinary day.

On your turns, Disbanded! will, for all intents and purposes, be "in effect" but not Ordinary Day so you can draw LOF to his figures and to your own regardless of hindering terrain. This also means that your opponent can draw LOF to your figures for Probability Control.

On your opponent's turns. Both Disbanded! and Ordinary Day are "in effect." Ordinary Day will trump the effects of Disbanded! so all TAs will also be in effect causing both teams to be in Stealth while in hindering.

While this has a positive effect on your team and not the opponent, the inverse would be true if you were both fielding teams with the Ultimates TA. In that situation, ignoring Ordinary Day would actually harm you and help the opponent.

"nerfing a BFC altogether" would be cancelling the BFC which is what Ordinary Day does. When something is cancelled, it is treated as if it is completely removed from play. When something is ignored, it is still in play, but you get to play as though it isn't.