However, I disagree with that assessment.
I'm not exactly sure why, but some possible reasons include:
1. I don't think "PIN" really means "personal identification number" any more. I think it refers to a new, specific complete concept. Something like "a low-entropy password used in conjunction with another means of identification". I think a PIN made of four letters would still be a lot more like a "PIN" than a "PIS". It's a fallacy to assume that ALL words automatically mean whatever they meant when they were first used, even if another meaning has been standard for 1200 years. PIN is much more recent, but if the new meaning is prevalent and useful and unambiguous, I don't think it's useful to cling to the old meaning.
2. "PIN" is short, and could be misheard. In the specific case of "please enter your PIN", no-one ever says anything else, so it doesn't matter, but I usually find saying a one-syllable word very loudly doesn't clear up any possible confusion, but "PIN number" is completely clear.
3. Being pedantic and knowledgeable about etymology is a stylistic choice, one which I'm naturally at home with. But I think it's a mistake to think that it's automatically better than learning language by hearing people use it. I think geeks are often marginalised in many circumstances, and assert their identity with this sort of grammar pedantry (which is a good thing). But it can lead to thinking that people who are not pedantic about grammar are less good, and feed into class prejudice that people who say "10 items or fewer" are somehow bad people :(
4. I'm stubborn. I express pedantry by choosing usages and using them specifically, even if other people disagree. I think there's nothing wrong with splitting infinitives, so I do so, even if the people I'm disagreeing with are themselves persecuted grammar pedants.
5. Redundancy isn't the be-all and end-all of language. In fact, it doesn't matter! I'm often non-redundant because I think it's funny or because compscis habitually express things with precision and little redundancy. But language is naturally redundant for good reason. There's no reason to avoid redundancy unless ABSOLUTELY NECESSARY, if it's often harmless and often useful. Clarity and unambiguity are more important.
But I'm not sure I'm right. Are there reasons I'm missing to be more pedantic?
 I also make mistakes, especially with spelling.
 Not doing so is a legitimate stylistic choice, even if I don't think it always makes sentences sound better, but not one that's BETTER than me.
 Eg "Did you receive this email?" "Mu"
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