I recently reviewed Lionel Shriver's book We Need To Talk About Kevin. A book in which the main element of the plot is clear from the start even though it's only described in detail near the end; but the ending chapters of the book contain two secondary plot twists that hit hard indeed, and leave the reader feeling rather conflicted at the end of the book (at least that's the effect they had on me). If those two twists were spoiled from the start, I guess one could argue the psychological depth of the characters would be enhanced, and the reader could watch out for indications of what was to come (this is of course assuming that the book or story is sufficiently well-written as to hold up to such closer scrutiny - in novels that's probably the case most of the time, but in TV or movies, I dare say it sometimes isn't). The reader would also be saved a nasty shock. But they'd have a less emotional, less spectacular reading experience. I guess it's a matter of preference, but imho most people like to get caught up in a book, and so want to read books unspoiled if they have the choice, rather than the more analytical, detached reading you do when you already know the big plot twists. At least the first time. You can always do the analytical reading on your re-read.