I think it is true that the experience of literature, or any storytelling, isn't spoiled by knowing how the story will end. I cheerfully reread books all the time, for the pleasure of re-entering the world and spending more time with the characters. In many cases, it's also to learn more about the book (beware of anybody who claims to understand Dhagren, or Ulysses, or Gravity's Rainbow after one reading). I enjoy reading good mysteries even when I already know who did the crime and why and how. (Sometimes I don't remember, of course, but that's a different question). Why would I ever want to stop visiting 221B Baker Street, or the brownstone on West 35th, or the city of Bellona, or Amber, or Dublin on that certain day 103 years ago?
But there is also a particular pleasure in reading something for the first time, not knowing what will happen on the next page. When I read The Green Mile the first time, in six monthly installments, I thoroughly enjoyed that month of trying to figure out why the hell the shoelaces were so important (which I never did figure out). Knowing that answer in advance would not have spoiled the book, but it would have spoiled that specific pleasure.
When people ask what U-town is about (and what "U-town" itself actually is), I try not to reveal very much, since I think that it is best to find yourself here, with Vicki, on the bridge, with no idea where you are or what is happening, and to discover the answers as she does. When Corwin wakes up in the hospital with amnesia at the beginning of Nine Princes in Amber, it's much more fun to learn who he is, and what Amber is, and who the nine princes are, as he does, rather than knowing it all in advance. Though of course, I re-read all the Amber books, and U-town, from time to time, too. But even there, part of the enjoyment is remembering that first reading (or, in the case of U-town, the writing), when I didn't know.
In any case, getting back to Harry Potter, the main purpose of the secrecy is not actually the protection of that "magic moment," it's the building of hype (hence the article I link to above). And it's working, since the article caught my attention, obviously, even though I've never read a single Harry Potter book, or seen any of the movies.
But, after all, if you're really trying to keep a secret, you don't describe your methods in such detail. But that wouldn't get you a 4-page article in Time.