Film critic Nathan Lee argues that, more often than not, spoilers are entirely justified: "To spoil or not to spoil involves larger questions about the role of the critic, the needs of the reader and the changes to both caused by the scale, speed and outlaw spirit of Web-based commentary...
"It’s silly to insist that the critic never spoil. In practice, spoilers can be irresponsible, motivated by laziness, vindictiveness or snark, but if the ambition to inform the reader outweighs the need to protect them, then spoilers are warranted on principle. The integrity of the critic doesn’t revolve around whether or not they’re willing to spoil, but why they chose to do so.
"Our obsession with spoilers has a diminishing effect, reducing popular criticism to a kind of glorified consumer reporting and the audience to babies. People outraged by spoilers should avoid all reviews before going to the movies or reading the book they’ve waited so long for, because the fact is all criticism spoils, no matter how scrupulous."
BTW: It's worth noting, by the way, that spoilers were being sprung long before the advent of the Internet. I still get angry -- well, OK, maybe not angry, but at least mildly miffed -- when I recall how I learned (accidentally and unwillingly) the identity of Luke Skywalker's father long before I got to see The Empire Strikes Back.