I understand where you're coming from, but the difference is that w/ Berserk/Defender, you're swapping your attack power for defense or vice versa. In theory, you're doing the same w/ Yonin. However in practice, you're swapping the only way for you to build Enmity(damage) for more Enmity. It'd be like Berserk lowering accuracy instead of Defense. The debuff basically counteracts the buff. And actually in the case of Yonin, the debuff outweighs the buff in regards to building hate.
The result is the ability is useless on anything where that accuracy is required. So it's basically just for soloing/duoing weaker mobs. If they actually want us to be a tank, we're going to need more than just a solo ability.