I understand what the Allowed Sites feature is, and why you would want to enable it. I'm just trying to understand why someone would NOT enable it. What is a scenario where you wouldn't want to do this? Is there some bug or inconsistent behavior with it? Are there webmasters for whom keeping a list of their domains would be inconvenient? And I only ask because Google isn't strongly suggesting or recommending that I use it, they are just offering up a feature that, to my inexperienced mind, seems like it should be the default.
People that have a ton of sites, its a pain to update. Also I get some of my blogs scrapped and if they want to scrape my adsense ads too thats fine with me as long as nothing weird happens.
It does make sense to use it, I think it just comes down to the fact that people are too lazy to do it? Which includes myself. It's just one of those extra features that really isn't necessary.