I've been thinking a lot about this matter. The situation I told, of an entire warband of Heartguards and a Blade Maiden Champion killed by a mirmadon in one turn because they dared to charge it, was something absolutely unacceptable.
Thinking about it, I think that the most realistic use of this ability (permanently braced) is as follows:
You need no wait counter to brace (we all agreed about that) BUT you are one f....in' warrior, not a warband, and neither you are Bruce Lee or a human machine gun, so: you will automatically brace the FIRST attack you'll receive in a turn. If I have a braced model, indeed, and he receives a charge, he strikes first. If a second model charges him, he receives the charge.
So for an Impaler, who has got a dwarf managing the balista and another one protecting-serving, the braced attack will be one; the first. For a goblin mirmadon, we'll have ONE braced reaction for EACH SIDE of the beast. Remember that the two spearmen are covering one side each, with their spears. So, if I charge with two models the same side, the braced reaction should affect the first only. If someone charges on the other side, there will be the second spearman bracing too, for the first attack only.
Playing like that would grant a great advantage to the model, but a realistic play too. You can't destroy a warband all alone if they charge you. It's ridiculous.
Please note that a Stygian priestess has the snakes to protect her. It is stated clearly that the snakes always attack first in EVERY situation she gets into close combat. I think that the possibility to hit ALWAYS EVERY MODEL FIRST should be written in a similar way, while Always braced means they are braced without going on wait, which means they work as every other braced model, except for this condition.
Let me know what you think about it.