Well, this is a question about grammatical terminology, not about grammar, or English grammar, as I understand it. I've looked at a bit of the prior discussion here, comments and answers, but I'm not replying or referring to any of that here. I'm afraid I'm also not replying or referring to @JK2's concerns, either. This post is strictly intended as an answer to the title question, more or less as stated.
What is the merit of calling a verb phrase a clause?
But let's leave out the "merit" part; that's a preachy word in my English -- substitute "purpose", since this is something linguists do on purpose. And let's also not limit the answer just to verb phrases and clauses; if possible, a general principle -- or at least some well-plumbed rules of thumb -- would be welcome.
In the case of the verb phrase and the clause -- the presenting problem, as it were -- the question to ask, once you're able to identify verb phrases reliably (which takes some practice), is
- What is a verb phrase, anyway?
There are a lot of things, and kinds of things, that are identifiable as verb phrases. A vast number of them. How do we learn to recognize them? Well, we tend to find them in clauses. Clauses are not really complete without a verb phrase, especially in English. In Even Cowgirls Get The Blues, Tom Robbins makes this point with the following statement:
Probably the simplest and most useful description of a verb phrase is
- A verb phrase is a clause that's missing its subject.
In other words, if you have a verb phrase, all you need to do is supply a subject, and you have a clause. And supplying a subject is never a problem -- we're good at that. That's why people continue to dangle participles and make dumb jokes -- they know their addressees will figure out what they want them to.
So here's the situation that puts a syntactician in. (One knows, of course, but doesn't particularly care, how The Oxford Dictionary defines phrase or clause; that's up to the syntactician to tell the dictionary, not the other way around.) The purpose of grammatical terminology is to make things easier to understand and remember. If verb phrases are associated with clauses, and one can always produce a clause with an appropriate subject for any verb phrase, why not say that verb phrases are just clauses with pieces missing? What would you do if you found a bird wing on the ground? You'd look around for the rest of the bird.
And so certain types of syntacticians (of which I am one) will tell you that every verb phrase in a sentence defines its own clause. Most of these clauses lose pieces because the pieces are predictable (by rule -- discovering these is what syntacticians do) and therefore recoverable (or else unimportant and therefore ignorable). This is basically what McCawley's grammar of English describes.
As it turns out, there are a lot of regularities. For instance, tensed English clauses have to have subject NPs present, to the point where dummy NP subjects like it and there are required, not optional, in many cases. On the other hand, untensed English clauses like infinitives, gerunds, and participles are frequently missing subjects and other chunks.
- Bill wants [for Bill] to win = Bill wants to win.
- Bill wants [for] Bill to win = Bill wants himself to win.
- Bill wants (for) Mike to win = Bill wants Mike to win. (optional for)
These are handy and pop up all over the place; think about the purpose infinitive, the relative infinitive, and all the gerunds and participles people use for all kinds of purposes. They all represent longer clauses, and these clauses can be exhibited as needed. For a syntactician, that means an easy way to relate and categorize a lot of different syntactic phenomena, all under one rubric. That's irrestistible.
And that's the title McCawley chose for his grammar of English:
One could go on in this vein for quite a while, but I'll content myself with the observation that this is the same purpose, and the same kind of thinking, that allows clauses to function as noun phrases, and as adverbs, and as modifiers in a noun phrase. You use the tools to hand.