Well, to be fair, you *can* tense up your abdominal muscles without yelling. And alot of people dont lack confidence - Theyre socially conditioned to not be loud and noisy. Which makes it 'uncomfortable' for them to *be* loud and noisy.
Of course you can. Yelling can be a tool to teach them that. We're talking beginners learning to yell. Not everyone automatically knows how their body works. Still sounds like the socially conditioned people you mentioned lack confidence.
A Chinese Martial Art Point of View
Can you imagine someone only being able to handle a hit when they yell..... that means I attack that person again immiediatly after they yell...when they inhale.... and they will have to inhale
Yelling and kia and all that has its place and is basic training for many arts....not the ones I train... but many.... just because someone does not or cannot yell has nothing to do with confidence
Whoever said we turn into noodles after yelling? If they had the confidence to defend themselves, they would be confident enough to yell. Of course that does not mean a person HAS to yell if they are confident. Imagine some parent that yells alot and the kid doesn't respond how the parent wanted and the adult is flummoxed. Obviously just being loud is not always enough to be effective.
The tightening of the abdominals is just a secondary consequence of the kiap which should also be happening when you exhale during a strike. The exhale, like the kiap, is a short sharp release of air (like a fart but from the other end). It's mainly for a distraction and to increase the amount of effort that is put into a strike (the same reason tennis players grunt when they hit the ball and weight lifters sometimes yell when lifting weights). If you can get a strike in when your opponent whilst they are inhaling you will do a lot more damage but it would be difficult get the timing right in the heat of battle.
We don't yell on every move. That would be strange. There is a lot of hissing or "ch"ing that can go on. Not necessarily with beginners, though. At some point they learn kiap and the hissing stuff. Beginners don't necessarily know about generating power with the way they breathe, the yell is A place to start them learning.
While timing your strike with an opponents exhale may be tricky, it's possible to do it by accident, or as a side effect of what your trying to do (still an accident). I already knew how yelling generates power, I didn't know the point the instructor made about the tightening having a protective function. Depending on which kids come to class on a given day, it can be easier to explain the protective function.
We're talking beginners here. I have in mind a particular beginning student, his kick would barely change the direction of a flying insect if he hit one. So light it barely could count as exercise. It's soooo hard to get him to yell. I don't expect the kids to kick the target out of my hand. Getting him to yell, to feel that power in his abs, his center, it might ground him enough to feel that he has more power and along with that a touch more confidence, and bring more ooomph into his kicks. Perhaps feeling that power, knowing it's there, would make him look less like a good target for bullies.
To the OP's question, when I started I only had a little trouble with yelling. Now I easily use it in classes. My favorite time to yell is once at the start of a sparring match. Now it's more of a self-preparation thing. Instead of running through lengthy mental list of things to prepare myself, the yell reminds me of those things in a moment. A bigger problem was being worried about the people watching. We were located in a mall and there was a long window to the hallway and people often would stop and watch. I got used to it, but I still like to practice in privacy. I still have trouble performing (as in practicing, haven't tried a competition for a while) in front of people sometimes.