All this got me curious - so I took a look at:
http://en.wikipedia.org/wiki/Special:Whatlinkshere/Template:Spoiler
One problem is it is distinctively patronizing to our readers - of course, if you read material under the heading 'plot summary' - you will be told (guess what) the plot. We don't need spoilers in such cases.
But among the more unexpected uses are:
A Biblical book:
Romeo and Juliet (they die)
Hamlet (he dies too)
My Fair Lady (she doesn't die)
Star Wars (cos someone doesn't know he's Vader's son? )
The Graduate
Casablanca, Emma (FFS!), The Hunchback of Notre Damme, The Wind in the Willows, the Grapes of Wrath, Lord of the Flies, Dorian Gray, The Taming of the Shrew, Snow White (?), Sons and Lovers, Tess, and Petronius' Satyricon (yes, that's a work from the 1st Century AD!)
And that's just with a quick skim.
Now, it is one thing for buffyfans and trekies to protect their secrets. There's a weak case for books just published and episodes yet unscreened in some parts of the world. After all perhaps someone searching us here has just come from some fandom site where spoiler warnings are expected. However, when this crap juvenile starts getting into our mainstream arts coverage - and particularly historical works - we just look bloody ridiculous.