Have you ever read a Sherlock Holmes story? I ask because of a specific technique that the author, Sir Arthur Conan Doyle, often used. As you probably know, Sherlock Holmes always rather miraculously solved very complex mysteries. What you may not know is that Holmes generally solved these mysteries at the very beginning of the book. It's not until the end that he reveals how he noticed mud on the pant leg, or scratches on the back of so-and-so's hand, or some other incredibly important clue that you, the reader, was never made aware of in the first place.
It never failed to piss me off, and I hope you'll pardon the language. The point is, maybe I could have solved the mystery as well. But nobody ever told me about the mud on the pant leg. The author chose to be ambiguous. And I don't think I'm the only reader out there who prefers that writers be straightforward instead.
Being ambiguous with the reader does serve a certain purpose, at times. Certain twists could never exist if you revealed all the information right away, after all. J. K. Rowling didn't reveal Snape's true nature for 7 books, for example. If we'd known about Snape from the first book, we never would have hated him and none of it would have felt the same. So, yes, holding back can serve your ends well when you're the author. But you have to be extremely careful about what you're holding back from the reader, and why.
When you think about it, a book is really just information. It's a lot of information about what happened, or what's happening, to a character or a group of characters. When you're selfishly hoarding some of that information, you're always running the risk that the reader is going to feel cheated by you. The reader is going to wonder why you didn't give them that information a long time ago. When you make a reader mad at you, you've put yourself in a pretty bad position. You've made it less likely that the reader is going to buy your next book. That's not where you want to be.
If you're being ambiguous, you'd better have a darn big payout for your readers. You've got to have a good reason for withholding. Give them an amazing twist, shock them, surprise them. But do something. Don't hold information back just for the sake of your plot. That's not a good way to make new fans who become dedicated readers.