When I'm in defensive stance, my role isn't to dps, my role is to soak up damage and try and control aggro. On test right now the single target AD3 @80 is 2500 threat, encounter is something like 1900-2400, and the proc is 750 threat when hit by a mob. Compared to the numbers floating by on the warrior class defensive stances, this is going to leave brig off-tanking as more or less impossible as threat transfer won't be available any longer and generating the amount of hate needed to hold aggro is... not going to be a brigand specialty, shall we say.
This very well could be as intended, I dunno. I viewed the rogue tanking as kind of the middle ground between avoidance and mitigation tanking, and we certainly don't have any real group aggro control as is - just hoping that perhaps something could be done about that.
It's not like many brigands tank that I know of, though. *shrug*
Oh, and on a side note, the 50% dmg reduction applying to auto-attack scales up in negative impact in a non-linear fashion as your MC/DA raise and you go mythical. For brigs like me w/40% MC 10% DA in defensive and fabled mythical, going from 800dps slashing to 400dps slashing of my 2k I parse isn't the end of the world - it's a reasonable sacrifice to make for some sort of advantage in the threat increasing department.