Given the way it's written, there's no argument to be made. In your example, the level 5 rogue with 3d6 sneak attack who uses the Two-weapon Fighting feat while flanking, hits with both attacks, and inflicts damage with both attacks gets a +6 dodge bonus to Armor Class for 1 round.
Perspective in necessary, however, when saying this option is too powerful. At low levels, this is a lot of resources to devote to a relatively benign trick that improves the rogue's otherwise low survivability (2 bad saving throws, multiple-attribute dependency, a nonspellcasting combat-oriented class with d8 HD and medium Base Attack Bonus) and forces the low-level rogue into taking the full attack action for maximum benefits. This means, given the general inability to sneak attack at range after the first round of combat, putting half the party into direct confrontation on opposite sides of the foe, thus often limiting the ranged attackers abilities to target the foe (e.g. a wizard's area attacks have to be carefully placed, an archer might still suffer at least a -4 penalty to attack rolls for his allies granting the foe cover). Further, just navigating into such a position, with Pathfinder's unforgiving stance on moving within enemies' threatened areas, can lead to a dead rogue before he's even had a chance to get the bonus.
At high levels, navigating threatened areas is extremely difficult, the number of feats devoted to the rogue's Two-weapon Fighting style goes from a lot to all, and the number of foes who are immune to sneak attacks--not because of their types but because of what they own, cast, or do--means the handful of times the rogue can use his trick, he'll have earned it, which is, I assume, what most folks want to happen anyway.
If you play with it the way it's written the Offensive Defense talent is nice occasional treat for a specific kind of rogue who desperately needs the assist.