Who's right?

[Bridget]

Do you agree with this?

"(...) when most speakers use a form that our grammar says is incorrect, there is at least a prima facie case that it is the grammar that is wrong, not the speakers."
CGEL

[Lucretia]

Bridget,
The debates would take months. In fact they have been going on for years. I think it's too vast and contentious an issue to start.

[Bridget]

Bridget,
The debates would take months. In fact they have been going on for years. I think it's too vast and contentious an issue to start.

I think it's a very important issue for ESL/EFL learners. I'm tired of being thrown to and fro by native speakers who can't agree upon whether an item of their language is acceptable or not.

BTW, do you agree with the topic statement? That's my only question here.

[Pete]

Do you agree with this?

"(...) when most speakers use a form that our grammar says is incorrect, there is at least a prima facie case that it is the grammar that is wrong, not the speakers."
CGEL
I agree with that statement.

Unfortunately, I don't think it resolves a lot of practical issues by itself. When you're speaking or writing, you have to take into account things like how formal the situation is and what dialect of English you are expected to use at the time. The authors of the grammar that you referenced were explaining the basis for selecting the structures to include in their grammar of English; they weren't saying that because a structure is grammatical, it is appropriate in all situations.

In a public place (like on a street corner or in a store), I often hear people speaking happily to each other without any thought of a problem with the other's speech, but using phrases like "Him and me are going to …" or "Ain't it?" Speaking like that in a job interview for a job like office secretary would probably prevent you from being hired. These people were conversing in a dialect other than standard English. There's nothing wrong with that, but it wouldn't make the grammar part of standard English, even if "most speakers" did it. (I don't know what the proportion is; I don't shift into that dialect myself.)

[Bridget]

Pete, the folks at CGEL were not referring to obvious items of non-standard English such as "Kim and me saw the accident", which they agree is non-standard, but more to examples like "It's clear whom/who they had in mind". Apparently, many prescriptivists would say that only the "whom" version is grammatically correct, while admitting that the "who" version is "sanctioned by usage". But, surely there is a case that the "who" version is also correct when used in the informal register. If most Standard English speakers are using that version, doesn't that make it grammatically correct?

And, as far as I've seen, there is no universal agreement on what consists Standard English. For example, "They invited my partner and I to lunch." is used by some Standard English speakers, but not by others. So, is it grammatically correct or not?