See http://forums.xkcd.com/viewtopic.php?f=9&t=6424&p=147306#p147306
In retrospect I'm not sure why no one did this before. The sample isn't exactly representative (since it is xkcd fora). But one interesting thing seems clear; the public prefers spoiler warnings on Wikipedia and uses them.
Maybe it is just me, but this seems like a pretty good argument for reinstating them, or at minimum undeleting the template and letting people use them where they judge to be more or less necessary.