If you have ever been to a Chiba Sensei seminar where he taught bokken you probably have tsuba on your bokken today. He does a technique (I believe its called tsuri otoshi) where the attacker does a shomen and you do a slightly quicker shomen while pulling your hips. When learning this you get hit a lot OVER the tsuba, I wouldn't even want to try it without a tsuba.
For what its worth Chiba Sensei doing tsuri otoshi, kiri otoshi against someone with no tsuba is not a good idea for the recipient of Chiba Senseis waza. Even with a tsuba one can easily get hit on the hand.
Of course you have to be able to respond to the attack and not rely on a tsuba. Learning to absorb the force applied by tori with ones total body is the trick.One needs to relax and try to neutralise the power of the strike.Even with a padded shinai one can still receive a stout whack on the knuckles if you are not careful.Motto-do not get hit!! Cheers, Joe.