From the school near my apartment a couple weeks ago came a continual "What's this?" Or at least, it was supposed to be "What's this?" Really, it was "Whaddis?" the ts becoming one sharp /d/, the h totally dropped so that the /th/ was entirely absent. It was the teacher asking this to a class who yelled so loudly in response that the answer was blurred in sound. How easy it is to get frustrated at mispronunciation, especially when it comes from someone who ought to know better or at least be more careful. But as I listen to many Westerners try to speak Mandarin, I hear the same kinds of mistakes. Indeed, my Chinese sounds so silly to me that I feel awkward using it in front of another Westerner, never mind a Chinese person.
Learning sounds out of context is clumsy, teaching sounds is difficult, and defending one pronunciation against another (vase: /vās/, /vāz/, or /väz/?) is sometimes pointless. This is not to excuse the, for example, /w/ that is often substituted for /v/. You model shitty language, you get shitty language.
It's amazing the number of self-professed experts out here. Language is so personal that to point out something as absolutely not true, or not always true, is to insult someone. It's amazing the rules I hear, the explanations made up to defend personal use or the dismissal of the things in the books. "Nobody talks like that." In my amazement—which I cannot escape, except when it's a constant, how much of a surprise is it?—I look everything up, have dictionaries on every conceivable piece of electronic equipment I can add a dictionary to. Things are said with such absolute certainty.