There is a big difference between can (am able to), should (what would be right) and ought (duty or correctness). We need to remember that.
Yes, a 15-year-old can belt the last G in “Defying Gravity”, but should she? Yes, you can teach a person to “scream” without being hurt, but is that what they ought to be doing? Yes, we can all sing all kinds of sounds but does that mean that we should all sing all kinds of sounds? We ought to know the why, when and what of some things before we rush to a quick decision.
Human beings make sounds. We cry, we laugh, we whisper, we yell. We moan, we whimper, we coo, gasp, and giggle. Almost everyone can sustain sounds and cover a certain number of pitches, go faster or slower and make them louder or softer. Doing this is part of normal human communication and does not generally take special skills. But just because we can make all kinds of sounds doesn’t mean we should make all kinds of sounds. It depends.
If you were to spend your days making every kind of sound there was to make, over and over, most people would think you were a fruitcake and your vocal folds would probably decide that they were having none of it. Just because you CAN doesn’t mean you should.
If you know that you are training your voice to do something that is out of the ordinary, and that you will have to do this on a regular basis, and that you might even be paid to do that, then you OUGHT to train yourself and your voice to be ready to do that in the best possible way. The training SHOULD help you to do what you want to do efficiently. If you know that you can scream once a night while performing in a play, then you should be doing certain things to make this easier, and you ought to realize this.
Every now and then someone thinks that everyone SHOULD learn to belt (like everyone should take a vacation to the Bahamas), but the reason for that is unknown. Why? Because it is possible? Yes, everyone who really wants to can learn to belt, but should everyone be a belter, should every voice learn how to belt just to know how? Is it so that if you do not belt, you are lacking in some way, vocally speaking? What about the reverse? Should you learn to sing classically because it will establish good vocal habits for any style? Just because you can learn to sing classically, should you? What if that is not what you want to sing? Do you do it because you ought to? Should you do something else just because, well, you can?
Sometimes teachers get ahead of themselves. They go from thinking classical singing is the “be all and end all” to thinking that everyone should learn to be a belter in the blink of an eye. Sometimes they forget that it isn’t always in the best interest of a 15-year-old to teach her how to sing that famous last “me” in Defying Gravity in Wicked, even if you (and she) can manage it. Sometimes they don’t connect the dots and look at the person, their long term goals, the feasibility of teaching a young person something that even an Olympic level vocalist is going to have to work hard to do, let alone keep on doing. Sometimes they lose track of the difference between can, should and ought. You can drive at 90 miles an hour, drunk, on a winding road in the rain at midnight, but that doesn’t mean you should. You ought to know better. You ought to know that just because you can doesn’t mean you should or you ought to.