OK, for most of my time as an Adsense publisher, I've never really used the Backup Ads option for anything but showing blank space as a form of backup ad when creating new ad units. To this day, I can't ever remember seeing a blank space because no ad was available. I'm discounting times when the ad script hangs and nothing shows up.
So, does anyone have evidence that Google ever shows a backup ad? My personal experience suggests that there is always some piece of junk to throw in the ad slot, even if it's completely unrelated to the content on the page and has nothing to do with the visitor's interests. Is this why people report irrelevant ads so often and is showing irrelevant ads just a way to make sure no other ad service gets a shot at the slot? Inquiring minds want to know. ;)
Hmmm. Subtitle should say "blanket" not "blanked" so you can tell I was thinking about blank spots when I wrote it. Or I'm a lousy typist. Take your pick.