The case for reading the books in published order includes the following:
1: The Lion is presented very much as the first of a series. It concludes with the words 'That is the very end of the adventure of the wardrobe. But if the Professor was right, it was only the beginning of the adventures of Narnia.' The 'second' book, Prince Caspian, is subtitled 'The Return to Narnia.'
2: The narrator of The Lion says 'None of the children knew who Aslan was, any more than you do.' But if 'you' are supposed to have read The Magician's Nephew, then you do know who Aslan was.
3: The charm of the opening of The Lion is spoiled if you already know, from Magician's Nephew, that the wardrobe is magical; that the Professor has been to Narnia, and why there is a street lamp in Narnia. Similarly, the 'shock of recognition' in Magician's Nephew is spoiled if you don't know the significance of the wardrobe.
4: Why should The Horse and His Boy, which happens during the final chapter of The Lion, be set after it? Could an equally valid case not be made for saying that it should be set after The Silver Chair where it is presented as a story-within-a-story?