[Andrew Whitehead]

How many people setting out to learn English have a clue, or even need to have a clue, about what 'medopassive', 'ergative', or any other of the strange linguistic terms invented by academics, means?

People 'setting out' don't need to know. People who want a deep understanding of how grammar works do need to know them.

I've lived quite happily and very productively through two millennia without ever either coming across or needing to use such terms.

I will probably get through my entire life in complete ignorance of many legal and surgical terms, but if I wanted to be a lawyer or a surgeon I would need to know them.

If you, personally, have no interest in grammar then you don't need to know what mediopassive or ergative verbs are, but what makes you think that you are in a position to criticise others who have more interest than yourself, and do want to learn?

I've lived quite happily and very productively through two millennia without ever either coming across or needing to use such terms.

This is an example of why grammar rules are needed: this sentence means you are several thousand years old...

If the person listening to what you say, or reading what you write, understands what you mean, it can't be wrong.

How do you know they understand? They may think they understand and even nod in agreement, but what they think you said and what you think you said can be two very different things.

Surely language is about getting your point across to others, not about how much your choice of style conforms to some unknown third party's ideas about 'correctness'.

One of the strengths of English is that it can be understood even when the grammar is mangled, but only as long as you stick to basic communication.

"Yesterday, I go shop" is as understandable as "Yesterday I went shopping."

If you want to convey only simple concepts you can get along by speaking bad English. If you want to convey more complicated ideas, where 'getting your point across' is not enough - you need to get it across clearly, precisely, and without ambiguity - then you need to learn the grammar. "He has learned to type" does not mean the same as "He learned to type".

The same argument you put forward here could be applied to any form of regulation.

" Surely driving is about getting from one place to another, not about how much your driving style conforms to some unknown third party's ideas about 'correctness'."

Should we abandon driving regulations? The logical flaw in this is that we drive safely, and communicate effectively, because we have regulations, and conform to a common idea of correctness.

You are making the mistake of viewing language as an isolated activity. It isn't, it is a social activity that only works if we all agree on what is correct and what isn't. To do that we need grammar rules that are written down and accessible to all. How is someone in Nigeria going to communicate effectively with someone in Canada if they start inventing their own local standards of correctness?

If you doubt this, then try to find an example of misunderstood communication that doesn't have a grammar mistake in it.