But someone armed only with a dagger have historically been able to defeat a heavily armed opponent - granted, there are conditions that need to be met, and it's not one-on-one combat. For example, Roman infantry troops versus the Persian deghan (the heavily armed 'knight' of that age); the tactics used was, let them charge a fortified position that had barricades, or troops with long pikes - once they killed the horses with projectile fire, the deghans were pretty much stuck on the ground, having lost the advantage of speed and momentum needed to break a group of well-disciplined foot soldiers. But these are special circumstances.

I agree that what we see in DA is unrealistic, though, but I do think there were 'dirty fighters', looked down on because they use poison and dishonorable tactics, like the suppa of Japan.