When a Couple Is Not a Few
The First Error
There are some issues of usage I find myself mentally correcting in the midst of conversation. I think content is more important than form: politeness and decency preferable to grammatical perfection. However, mental correction is always permitted. Whether only in Canada or the English world over, I can't be sure, but here at least many misuse 'couple' to mean 'few'.
For instance, someone might say,
"It usually snows a couple of times each December."
The word 'couple' here is intended with the vagueness of 'few.' The misunderstanding is that 'couple' is a vague term, referring to an anomalous collection of a small number. In actuality, of course, that number is a fixed small number: two only. The speaker of the above sentence couldn't be saying it snows only twice each December, but that is what the sentence, strictly speaking, would express.
The Second Error
Even knowing 'couple' refers to exactly two objects does not prevent error. Words don't have meanings, they have usages. Dictionaries do not tell what a word should mean, but how a word is used in a sentence. We do not think in words, but in ideas; and ideas are captured not by words, but by sentences. Dictionaries are guides to sentence construction. When two words appear that seem to be used the same way, usually there is a connotative distinction to be drawn. 'Couple' is not ideally used wherever 'two' may be used. The nuance it captures in general usage is important for expressing our ideas in sentences. 'Couple' is distinguished from 'two' by the implication of a connection between the two objects to which it refers.
"Only a couple of people know the secret," could be rendered, "Only two people know the secret." Perhaps the aesthetic value of a few extra syllables guides the word choice here, but were that not the case, what might be the significance? 'Couple' would normally be referring to an actual couple. A couple of people who know the secret would be people who are related to one another, a romantic couple or coupled by virtue of the secret. To say 'two people know the secret' would not imply this connection. They would be people with no relationship to one another, or at least not for the purpose of the discussion.
A Couple of Additional Points
Similarly, 'a pair' also refers to two objects. It also refers to two objects in a relationship. One speaks of a pair of shoes or a pair of gloves, but not a couple of gloves or just two gloves. The connotation of 'pair' is one of necessity. The two is defined by their being two and in this sense the duality is a necessity. For instance, a 'pair of twins' is so called because their very nature as twins depends upon the duality. One could well refer to them as a couple of siblings, but when referring to them as twins, they are not merely a couple, but a pair; they together form the classification entity that is the twinship.
This necessity is not contained in the notion of a couple. The couple's couplehood is contingent, a property that might just as well have not been acquired or have been acquired with different entities. Gloves are always made in pairs and are necessarily paired, but married couples could just as well have never met.
Ultimately, it little matters in the grand scheme of things whether you say 'pair', 'couple', or 'two'. Usually the casual listener can figure out whether 'few' is intended instead of 'couple,' or whether 'couple' is being used loosely. Concerns from nuance, elegance, aesthetics, or even the comic value of the word choice all come into play. However, awareness of the connotative distinctions enables one to use or misuse them more effectively.