Yelling: not all bad

Every parenting book I read starts off with the chapter that goes on at great length about the evils of poor parenting. The book I bought on yelling went on and on about why it is bad. I frankly do not understand why they are doing this. Obviously, I already figured out that bad parenting is...well...bad. Obviously, I want to change. Need proof? I bought/rented/borrowed your book and am reading it! The only reason I can come up with is that the authors are trying to strengthen readers' resolve to improve and possibly bring in some scientific studies laypeople are not aware of.

I will avoid the reason why one shouldn't yell at her kids. They are too numerous and too obvious to recount. I will only list one reason that I haven't seen in the literature so far. And this reason is as follows - yelling works only when done occasionally and only in limited circumstances. If done often, it becomes white noise, and kids no longer pay much or desired attention to it. Yes, you have heard this before, but please bare with me.

There are situations when yelling is absolutely necessary and getting your child's attention RIGHT NOW is of utmost importance. They are the times when a child is doing something dangerous, but is out of parent's immediate reach. Putting something in the electric outlet, stepping on a broken glass, carelessly crossing the street when the car is approaching - these are examples that are coming to mind. If I freak out and break into a yelling tirade when my son pours apple juice all over the floor, even on purpose, and over other similar mischief, yelling becomes equivalent with mother trying to stop fun activity. So when he eventually reaches for a sharp or hot object, he will much less likely heed the warning, "DON'T TOUCH THAT!" no matter how loudly uttered. Ultimately, I had used up my power of a scream on things of little importance.