I'm another abridgement snob. Someone mentioned The Swiss Family Robinson? See, I think the archaic, cheesy, formal language is the most worthwhile and delightful aspect of the book. I can't imagine a simplified version doing justice to the "distended maw" of the snake that swallows the donkey, for instance. It doesn't seem problematic to just let the kid wait until he's old enough to read the real version, especially as there are heaps of great books at every reading level. So it's not like he'd be spending the extra years with nothing to read.
I also believe it's good for kids to read, or have read to them, books that are "too hard". Helps stretch their understanding of language. And kids can usually appreciate the books on some level, if they're well-chosen. DD's three, and into Beatrix Potter at the moment. I initially felt a bit silly reading her all the quaint phrases I knew she wouldn't understand - some of the comedy goes way over her head, and she has no idea what rabbit-tobacco is; nor do we use the word "presently" in real life. And so on. But she likes the stories - mainly for the pictures and the danger elements, like Peter and Benjamin Bunny being sat on by the cat - and as we keep reading the stories over the years, her language will catch up.
I will make an exception, though, for (oddly enough) Shakespeare. Even for well-read adults Shakespeare can be tough going, and it doesn't help that plays are meant to be watched rather than read... but when you watch the plays, they can move too fast for you to really appreciate the puns and turns of phrase. So it's handy to go into a Shakespeare play with a rough idea of the plot, and which characters are comic and which are evil and so on, just so you know what to look for. So a really good adaptation of the stories can be worthwhile, I think. Charles and Mary Lamb do a good one - they don't try to imitate "Shakespearean" language, but they tell the stories in a very simple, easy-to-understand way. The adaptation definitely shouldn't replace Shakespeare, but it's a good start, and dissimilar enough from the real works that kids won't feel like they've been there, done that when it comes to reading the actual plays.
On a vaguely related topic, we bought DD a children's Bible for Christmas and it's awful. The pictures are good and it has a wide range of stories, which is why we bought it... but the stories are tamed down to the point of incomprehensibility. It's like the authors went "Oh, we should put in Samson, he's a famous character... ooh wait, that's a bit sordid" and gradually left out more and more of what actually happened to Samson, until they were left with something along the lines of "Samson was a very strong man. He could break ropes when people tied him up. He even pushed down a palace to punish God's enemies". I kid you not. No Delilah, no "out of the eater something to eat", no eye-gouging. Which, I mean yes, a bit rough for the preschool crowd, but if the story is inherently too gory for tots, why include it at all? Better leave it out than leach all the story out of it until it's totally pointless. It's like that atrocious Disney version of Notre-Dame de Paris.