'My point is that we should value spelling and grammar and not treat them as optional extras.'
Who does? Schools which follow the National Curriculum HAVE to teach these things.
'If we say they don't matter, we are holding people back.'
Who does say they don't matter? But there's a difference between teaching students about the differences between Standard and non Standard English and when it is appropriate to use each and teaching them that the way most of them speak and write is incorrect but there is a correct way (which there isn't Standard English is just a dialect which has gained prestige). For example, a Scottish student may use the word 'wee' for 'small' and 'bairn' for child. This is not incorrect it is just part of his or her dialect.
'I'm not blaming teachers, per se, but I do think an examination system which doesn't value spelling and grammar is stupid and damaging.'
I think you've been slightly misled by the article. Marks ARE allocated for spelling and grammar on GCSE English papers and assessment criteria means students are given credit when they use 'appropriate discourse markers' or 'a variety of sentence structures' for example. Also, if spelling and grammar impede meaning students are marked down as you would expect. I have had the great privilege to mark 300 English GCSE scripts .Usually there's a link between good spelling and students who are good at English generally. Other subjects also allocate marks for spelling and grammar - certainly History and Media Studies do. A student who is awarded an A* in English is very good at English even if his or her spelling isn't perfect I can assure you (though no doubt you will know exceptions).
Also, I note nobody has responded to my point about Shakespeare. He might well fail his English GCSE if spelling was the main criterion. Some of you sound like a bunch of Gradgrinds to me.
'And I'm afraid it's simply not true that an inability to spell or write clear English doesn't hinder understanding. What about censure/censor, complaisant/complacent, councillor/counsellor, diffuse/defuse, just for starters?'
There would be very few cases where you couldn't work out what someone was communicating through context. We all make these kinds of mistakes when we're tired etc.
As I say, nobody these days would find it difficult to understand 'C U L8R' but it certainly doesn't use standard spelling.
Of course, we need to communicate clearly and of course there are occasions where it is inappropriate to use dialect or non-standard English but this emphasis on 'correct' spelling as though there were such a thing often has more to do with snobbery than clarity.