Again, I'm not saying that you can't damage the person using a wooden sword, nor that the damage is less devastating. But, if you hit someone's unprotected wrist full-force with a bokken, their wrist break. With a katana, you have a good chance that they will lose the hand. Are either less immediately devastating in melee? No. BUT with a bokken, you're also considering that it can be used on a backswing, it's a lot easier to not deal deadly damage and you have to worry a bit about the fact that it is easier to break than a metal katana.