I'm in the "as long as children are reading camp"
I have a dyslexic child with visual stress. The books I raced through with delight at his age, The Chronicles of Narnia, The Secret Garden, The Railway Children etc would give him a thumping headache before page 5 due to deciphering the wall of squirming text, shuffling around the page. If I want to ensure that he never turns a page for pleasure, well-respected, intelligent classics are the perfect method.
So I indulge what he can cope with. Captain Underpants, Horrid Henry, Diary of a Wimpy Kid. Crude, funny, minimal text, chunky layouts and simple cartoons that look like he could have drawn them (because he's not going to be celebrated for realistic drawing, and it hurts him when he holds a pencil, so it's good to see his style being validated). They don't stretch his intelligence, but they can be quite fun to read with silly voices.
I've had 12 year old boys brag to me proud that they've got that far through life without reading a whole book. They've skipped pages here and there, gazed vacuously at pages under the pretence of reading. I would much rather a child enjoys something crude, silly or over-popular than a child who does not read. So if David Walliams is accessible to these types of children (usually boys) that is a good thing. Classics have their place for the individuals who enjoy them.
Harry Potter is often criticised for writing quality, and after reading aloud, some sentence structures are a bit labourious. But they're fun, engaging, cleverly structured and thought provoking. I was the wrong age when they came out, and found the band-waggon when the films came out. I'd been in a reading drought, struggling in the gap between children's fiction and enjoying adult fiction. Harry Potter ended that drought and taught me that my main love is YA fantasy. Escapist, not too dry and serious. Yes my A-level English Lit is squandered, but I still read for pleasure in a world of so many distractions from the humble book.
Reading is good. Even if the author is a middling quality celebrity.