Under normal conditions, sure. But if you find yourself on ice, or hydroplaning, or driving through Death Valley, or running on empty while stuck in traffic, a basis of understanding of the physics of automobiles will absolutely help you out. By willfully ignoring the basic function of the automobile you are permitting yourself to be a victim of circumstance.

Living in Massachusetts, I've been driving in snow and ice as long as I've been driving. I'm also no stranger to hydroplaning. I didn't learn how to drive on slippery surfaces or regain control when hydroplaning by studying a manual, though, I learned to do it by driving in snow and rain.

I don't mean to suggest that you don't need to know how often your oil needs changing or to have some basic idea about, say, keeping your engine cool. I don't really know how it's all put together, though. I know there's a fuel injector and an engine block and some cylinders and an alternator but I couldn't tell you how they interact with one another exactly.

If I did plan on heading out into the desert it might not be bad to bring someone a bit more knowledgeable along. It doesn't affect my ability to drive well, though. Practice and knowledge of the road is what I'd say has more to do with being a good driver than being able to change a muffler. There are mechanics that hog the passing lane and don't use blinkers.

There's a major difference between understanding the inner workings of your own vehicle and knowing about standard maintenance and driving in hazardous conditions.

A basis of understanding is always better than a basis of mystery.

I absolutely agree. I'd say that the most important thing, though, is to develop a love for language. Parts of language should absolutely be taught, but passion for language is more important and will impact not only what they learn in school but what they learn as adults. Like you say, we can always look up the parts again later if we need them, but something has to encourage us to give a damn in the first place.

But if you can't say it according to the agreed-upon ground rules of written or spoken communication, no one will listen to you.

Again, absolutely true. As you say, though, the rules are different in different contexts. Spoken communication is also still much more prevalent than written communication. Most of the interactions you have, if you don't spend loads of time talking on the internet or have a job involving a lot of writing, are going to be influenced by your ability to speak rather than your ability to write. Most people aren't working in offices, at least not in the US. If you work in transportation, retail, or any sort of service industry profession you're not going to be typing out long reports that have a huge impact on your day to day interactions. Even if you are in an office whether your writing ability matters significantly or not depends on the industry you're in and your specific job.

This too I'd argue has more to do with practice than being able to explain the rules. A love of language is fundamental to getting sufficient practice in as an adult.

What I ran into, though, and maybe this is just because I had shitty English classes, was the presumption that these contextually appropriate prescriptions were a matter of correctness. There's a big difference between saying there's a single way that English should be and saying that there are certain situations in which English should be used in certain ways. One is strategic and intelligent, the other is oversimplified and frankly wrong.

I think we're mostly on the same page here aside from maybe a confusion about dismissal of the rules as useless versus not letting one style guide be an all-encompassing prescriptivist stance on all valid forms of English.