I have to say on the whole I agree - and I do have a child. Just one, a daughter, who is a lovely person with whom I get on really well - so that may make it easier for me to agree with you!
We both overhear parents talking to their kids on the bus, say, or in a supermarket, and wince - at times it sounds like the parents actively dislike their kids!
I realise this sounds judgemental and potentially even offensive, and it's not my intention. I am talking mainly about extremes, when people tell their children off with real malice.
I do think that modelling respectful and kind ways of talking is really important, if you want your kids to grow up respectful and kind.
But I have never had the (domestic) challenge of coping with several kids at once, or with a badly behaved child. So I guess I don't know what that would drive me to.
I am a teacher in secondary school, and while I try, again, to remain respectful and kind at all times, I do occasionally snap, have even raised my voice on occasion, when things have been particularly stressful. I do always try to apologise if I think I have been genuninely unfair.
(And I also think that there are certain extremes of behaviour - violence or extreme verbal cruelty to another child, for example - that genuinely warrant a verbal expression of outrage/anger - though I know not all would agree).