If the story poses a puzzle for the reader, and the solution to the puzzle is given further down the plot, then spoilers can in fact reduce the enjoyability of the story. In Death Note, you can actually discover the flaw in the character's plan yourself if you pause the video and and think for a bit (although it helps that the protagonist's intelligence is a bit...uneven. Most real people aren't simultaneously stupid and smart like that). It's also fun to arrive at the best possible strategy for each character...it's pretty satisfying when you and the character independently arrive at the same conclusion.

This only applies to very tightly written stories of course.