It isn't necessarily to highlight and thereby expose someone's error. Were I to quote someone whose words include an error and I corrected that error, that would be patronising in the extreme. If I were to quote someone, knowing that the quote included an egregious error but I didn't point out the error via [sic], it could look like I was condoning the error, or worse, didn't know that the error was there. [sic] is relatively subtle, is time-honoured and is not at all vulgar.