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.
Hear, hear! I've been removing spoiler tags from plot summaries in articles I'm involved in. I haven't quite got up the nerve to remove them from articles I haven't edited before, though... almost did earlier today.