Guys,
Your in your car and it goes into a skid.. do you yell as you try to correct? (ok, maybe you do as you are about to hit that 18 wheeler.)
When you pick up heavy furniture, do you yell as you lift? "Aaand, UUUP!" With a few grunts.
When you hammer a nail, do you yell? Hammering a nail vs a strike? One is all about precision and doesn't require much force, the other is about a powerful hit directed to a certain part of the body.
Yell as you chop wood? On a very hard swing, yes.
Yell as you life weights in the gym? Given the nature of it being a gradual progressive thing, I usually let out a grunt.
Ok, do you hear boxers yell when they fight? I don't watch much boxing, but I've heard lots of grunts and light yells.
Do Muay Thai fighters yell when the hit? Never watched Muay Thai sparring.
Did Chuck Norris and Joe Louis yell when they fought in tournaments? Before my time.
Ok, so you get the point, right? I've never yelled in a fight and while I'll yell to please the instructor, I don't put all that much stock in it.
Deaf
I think you're missing the purpose of a Kiai. We're taught to do it when moving into positions and on final strikes in parts of our forms (in Ki Cho Hyung Il Bu, for example, we kiai on our third strike when doing our three punches). I've unfortunately had to use TSD outside of the dojang, and a strong kiai does help add more force behind a strike.
Now if you'll let me ask you this,
When you're testing and you're required to break cement blocks, are you going to kiai? Or will you just let out a silent exhale?