There have always been only two reasons for seeking opt-ins—supposed legal protection and to negate future customer backlash. As a practical matter, opt-ins help out in neither situation. Let's start, though, by admitting there are two types of opt-in: knowingly/deliberate, and what retail chains use.
The typical shopper opt-in comes in the form of a line buried inside a legally dense and overly long Terms And Conditions document or maybe in a small-print reference on a mall entrance sign or as a "Click To Accept Our Restrictions" box on a mobile app. Rarely are shoppers even aware of all the provisions they have agreed to, let alone consciously weighing the options and making a decision.
The alternative is simply not viable in retail, where it's a one-on-one discussion with the customer in which the option is slowly and fully explained and the shopper is encouraged to ask questions to prove comprehension.
Given the way these opt-ins happen, what type of legal support do they offer? Not much at all. If consumers choose to sue and if the case ever gets to trial, a retailer will likely have to defend this opt-in in front of a jury of other shoppers who themselves have clicked off on hundreds of permissions. They will have no trouble believing the consumer plaintiff never knowingly told the chain it could track her in that way. It may not be fair—"Look, she signed. Right there. Plain as can be"—but juries typically rely on their own experiences, when applicable.
So what about the other type of opt-in protection—the protection from the shopper's pushback? No defense there, either. Do you actually think a shopper who is furious when she discovers how her daughter is being tracked will hear that she agreed to do this two years ago by clicking on a checkbox and will suddenly be OK?
That said, opt-ins do little to no damage, cost virtually nothing and do keep Legal happy. Just don't fool yourself that they provide any protection. The only protection is to not do anything you wouldn't want blasted on the front-page of the local newspaper.