Prescriptive grammar, pff.
Any grammar is designed to be deficient, as you say. I don't usually see it brought up in regards to language learning, because you kinda have to be practical there and I hope most sane people agree. It's not a useful lens to view language through otherwise, especially when people and institutions try to slam their normative dick on the table.
That said there are a lot of things which stir the curmudgeon in me, too, like "gonna wanna do" and "with no". Or the wave of transcription fuck ups (which I feel comfortable calling mistakes) like "could of". I hope it's just orthography, anyway. I don't have to like the changes to accept change, but please not in that direction...
Other things just have my ears piqued, because I've learned them differently not too long ago, like "sneaked". Desperately awaiting the first "swimmed" from an adult.