> they are also everywhere so they must be effective to some degree?

I wouldn't assume that. I would bet most people enter a fake email address just to make the thing go away because they don't know you can just close most of them, or don't realize the passive aggressive "No I don't want great deals sent to my inbox every day because I hate great deals" text link is a button they can press to dismiss the dialog because they don't read it and it intentionally doesn't look like a button to discourage users from pressing (or reading) it.