G'day Gabe,
OK, here's when they will be useful. I want to know the background of the Movie X before I go out to see it. You know, the actors, filming locations, etc. But not the plot. Do I not deserve to know what will spoil my fun, even when there is a perfectly functioning and unobtrusive system established for that purpose? ~~~~
You can get all that from IMDb, without the risk of your eye accidentally picking up information you don't want.
I wouldn't look at Wikipedia if I wanted to avoid a spoiler, whether there was a spoiler warning or not. The warning is not effective. It doesn't assist those who you think will want it, and it serves only to annoy those who don't want it.