If you look for actual logical consistency between different types of speech like you’re doing here, you’re on a fool’s errand.
First, the language itself contains many inconsistencies. Why, for example, does “ough” sound different in cough, enough, through, thought, and drought?
Second, the fact that “couple” means two doesn’t mean that they are used identically in sentence construction. “Pair” also means two - would you say that you bought a pair shoes, or a pair of shoes? A “brace” is also two of something, and the correct usage is “a brace of.” A duet suggests two, as does a duo, and yet, because they are nouns and not adjectives (just like couple) convention suggests the need for an intervening preposition if we’re going to attach them to some particular things.
Right, but it seems to me that it’s important to draw a distinction between speaking and writing, especially more formal writing. For example, when people say “would have” in regular speech, the speed of the conversation often makes it sound like “would’ve,” and a lot of people also hear it as “would of.”
That’s completely fine. Unless you’re delivering a really formal speech, we all understand that everyday conversation often involves elisions and apparent contractions that we would not make in formal writing.
But while i would never correct a student who says “would’ve” in conversation, even if it sounds a bit like “would of,” i still do correct them when they write “would of” in an essay or some other piece of work that they submit in my class.
But “couple” versus “couple of” is not quite the same thing, because it’s clearly used intentionally (not just as a sort of accidental speaking elision) in both spoken form and in written form. I have no problem with “pairsa” or “coupla” in everyday speech, because they’re products of the vagaries and imprecisions of off-the-cuff speech. I’d be willing to bet, though, that neither of you would write those things in, say, a job application.